Method for manufacturing nitride semiconductor substrate, nitride semiconductor substrate, and laminate structure

ABSTRACT

A method of making a semiconductor including a step of preparing a base substrate; a first step of epitaxially growing a single crystal of a group III nitride semiconductor having a top surface with (0001) plane exposed, directly on the main surface of the base substrate, forming a plurality of concaves composed of inclined interfaces other than the (0001) plane on the top surface, gradually expanding the inclined interfaces toward an upper side of the main surface of the base substrate, making the (0001) plane disappear from the top surface, and growing a first layer whose surface is composed only of the inclined interfaces; and a second step of epitaxially growing a single crystal of a group III nitride semiconductor on the first layer, making the inclined interfaces disappear, and growing a second layer having a mirror surface, and a semiconductor made thereby.

The present disclosure relates to a method for manufacturing a nitridesemiconductor substrate, a nitride semiconductor substrate and alaminated structure.

DESCRIPTION OF RELATED ART

A conventionally known technique is as follows: a group III nitridesemiconductor is further grown on a main surface whose low index crystalplane is a (0001) plane, using a substrate comprising a single crystalof a group III nitride semiconductor as a base substrate (seedsubstrate). According to this technique, at least one nitridesemiconductor substrate can be obtained by slicing a crystal layer grownto a predetermined thickness (for example, Patent Document 1).

PRIOR ART DOCUMENT Patent Document Patent Document 1: Japanese PatentApplication Laid-Open Publication No. 2013-60349 SUMMARY OF THEDISCLOSURE Problem to be Solved by the Disclosure

An object of the present disclosure is to improve a crystal quality of anitride semiconductor substrate.

According to an aspect of the present disclosure, there is provided amethod for manufacturing a nitride semiconductor substrate using a vapordeposition method, including:

a step of preparing a base substrate comprising a single crystal of agroup III nitride semiconductor, and having a mirror main surface whoseclosest low index plane is a (0001) plane;

a first step of epitaxially growing a single crystal of a group IIInitride semiconductor having a top surface with (0001) plane exposed,directly on the main surface of the base substrate, forming a pluralityof concaves composed of inclined interfaces other than the (0001) planeon the top surface, gradually expanding the inclined interfaces towardan upper side of the main surface of the base substrate, making the(0001) plane disappear from the top surface, and growing a first layerwhose surface is composed only of the inclined interfaces; and

a second step of epitaxially growing a single crystal of a group IIInitride semiconductor on the first layer, making the inclined interfacesdisappear, and growing a second layer having a mirror surface,

wherein in the first step, a plurality of valleys and a plurality oftops are formed on a surface of the first layer by forming the pluralityof concaves on the top surface comprising a single crystal and makingthe (0001) plane disappear, and

when observing an arbitrary cross section perpendicular to the mainsurface,

an average distance between a pair of tops separated in a directionalong the main surface is more than 100 μm, the pair of tops beingclosest to each other among the plurality of tops, with one of theplurality of valleys sandwiched between them.

According to another aspect of the present disclosure, there is provideda nitride semiconductor substrate having a diameter of 2 inches or moreand having a main surface whose closest low index crystal plane is a(0001) plane,

wherein X-ray locking curve measurement for a (0002) plane diffraction,which is performed to the main surface by irradiating with (Cu) Kα1X-rays through a two-crystal monochromator of Ge (220) plane and a slit,reveals that:

FWHMb is 32 arcsec or less, and

difference FWHMa−FWHMb obtained by subtracting FWHMb from FWHMa is 30%or less of FWHMa,

wherein FWHMa is full width at half maximum of the (0002) plane when aslit width in ω direction is 1 mm,

FWHMb is full width at half maximum of the (0002) plane when a slitwidth in ω direction is 0.1 mm, and

a diffraction pattern when the slit width in ω direction is 1 mm, has asingle peak.

According to further another aspect of the present disclosure, there isprovided a nitride semiconductor substrate having a diameter of 2 inchesor more and having a main surface whose closest low index crystal planeis a (0001) plane,

wherein observation of the main surface of the nitride semiconductorsubstrate in a field of view of 250 μm square using a multiphotonexcitation microscope to obtain a dislocation density from a dark spotdensity, reveals that:

there is no region in the main surface where the dislocation densityexceeds 3×10⁶ cm⁻², and a region having a dislocation density of lessthan 1×10⁶ cm⁻² exists in an area of 80% or more of the main surface,and

the main surface includes non-overlapping 50 μm square dislocation-freeregions at a density of 100/cm² or more.

According to further another aspect of the present disclosure, there isprovided a laminated structure, including:

a base substrate comprising a single crystal of a group III nitridesemiconductor and having a mirror main surface whose closest low indexcrystal plane is a (0001) plane;

a first low oxygen concentration region provided directly on the mainsurface of the base substrate and comprising a single crystal of a groupIII nitride semiconductor;

a high oxygen concentration region provided on the first low oxygenconcentration region and comprising a single crystal of a group IIInitride semiconductor; and

a second low oxygen concentration region provided on the high oxygenconcentration region and comprising a single crystal of a group IIInitride semiconductor,

wherein an oxygen concentration in the high oxygen concentration regionis higher than an oxygen concentration in each of the first low oxygenconcentration region and the second low oxygen concentration region, and

when observing an arbitrary cross section perpendicular to the mainsurface,

an upper surface of the first low oxygen concentration region has aplurality of valleys and a plurality of mountains, and

an average distance between a pair of mountains separated in a directionalong the main surface is more than 100 μm, the pair of mountains beingclosest to each other among the plurality of mountains, with one of theplurality of valleys sandwiched between them.

ADVANTAGE OF THE DISCLOSURE

According to the present disclosure, a crystal quality of a nitridesemiconductor substrate can be improved.

FIG. 1 is a flowchart illustrating a method for manufacturing a nitridesemiconductor substrate according to an embodiment of the presentdisclosure.

FIGS. 2(a) to 2(g) are schematic cross-sectional views illustrating apart of a method for manufacturing a nitride semiconductor substrateaccording to an embodiment of the present disclosure.

FIGS. 3 (a) to 3 (c) are schematic cross-sectional views illustrating apart of a method for manufacturing a nitride semiconductor substrateaccording to an embodiment of the present disclosure.

FIG. 4 is a schematic perspective view illustrating a part of a methodfor manufacturing a nitride semiconductor substrate according to anembodiment of the present disclosure.

FIGS. 5(a) to 5(b) are schematic cross-sectional views illustrating apart of a method for manufacturing a nitride semiconductor substrateaccording to an embodiment of the present disclosure.

FIGS. 6(a) to 6(b) are schematic cross-sectional views illustrating apart of a method for manufacturing a nitride semiconductor substrateaccording to an embodiment of the present disclosure.

FIG. 7(a) is a schematic cross-sectional view illustrating a growthprocess under a reference growth condition such that an inclinedinterface and a c-plane are neither expanded nor contracted, and (b) isa schematic cross-sectional view illustrating a growth process under afirst growth condition such that the inclined interface is expanded andthe c-plane is contracted.

FIG. 8 is a schematic cross-sectional view illustrating a growth processunder a second growth condition such that the inclined interface iscontracted and the c-plane is expanded.

FIG. 9 (a) is a schematic top view illustrating a nitride semiconductorsubstrate according to an embodiment of the present disclosure, (b) is aschematic cross-sectional view taken along m-axis of the nitridesemiconductor substrate according to an embodiment of the presentdisclosure, and (c) is a schematic cross-sectional view taken alonga-axis of the nitride semiconductor substrate according to an embodimentof the present disclosure.

FIG. 10 (a) is a schematic cross-sectional view illustrating X-raydiffraction with respect to a curved c-plane, and (b) and (c) are viewsillustrating a fluctuation of a diffraction angle of a (0002) plane withrespect to a radius of curvature of the c-plane.

FIG. 11 is a view illustrating an observation image obtained byobserving a cross section of a laminated structure of an example using afluorescence microscope.

FIG. 12 (a) is a view illustrating a normalized X-ray diffractionpattern of the nitride semiconductor substrate of an example when anX-ray locking curve is measured with a different slit, and (b) is a viewillustrating the normalized X-ray diffraction pattern when the samemeasurement as in the example is performed for the base substrate.

FIG. 13 (a) is a view illustrating an observation image obtained byobserving a surface of a laminated structure of Experiment 2 using anoptical microscope, and (b) is a view illustrating an observation imageobtained by observing the surface of the laminated structure ofExperiment 2 using a scanning electron microscope.

FIG. 14 (a) is a view illustrating an observation image obtained byobserving an M-cross section of the laminated structure of Experiment 2using an optical microscope, and (b) is a view illustrating anobservation image obtained by observing the M-cross section of thelaminated structure of Experiment 2 using a scanning electronmicroscope.

FIG. 15 (a) is a view illustrating an observation image obtained byobserving a-cross section of the laminated structure of Experiment 2using an optical microscope, and (b) is a view illustrating anobservation image obtained by observing a-cross section of the laminatedstructure of Experiment 2 using a scanning electron microscope.

FIG. 16 is a view of observing a main surface of a nitride semiconductorsubstrate of sample 1 using a multiphoton excitation microscope.

FIG. 17 is a view of observing a main surface of a nitride semiconductorsubstrate of sample 1 using a multiphoton excitation microscope.

FIG. 18 is a view of observing the main surface of the nitridesemiconductor substrate of sample 1 using a multiphoton excitationmicroscope.

FIG. 19 is a view of observing the main surface of the nitridesemiconductor substrate of sample 1 using a multiphoton excitationmicroscope.

FIG. 20 is a view of observing the main surface of the nitridesemiconductor substrate of sample 1 using a multiphoton excitationmicroscope.

FIG. 21 is a view of observing the main surface of the nitridesemiconductor substrate of sample 1 using a multiphoton excitationmicroscope.

FIG. 22 is a view of observing the main surface of the nitridesemiconductor substrate of sample 1 using a multiphoton excitationmicroscope.

FIG. 23 is a view of observing the main surface of the nitridesemiconductor substrate of sample 1 using a multiphoton excitationmicroscope.

FIG. 24 is a view of observing the main surface of the nitridesemiconductor substrate of sample 1 using a multiphoton excitationmicroscope.

FIG. 25 is a view of observing the main surface of the nitridesemiconductor substrate of sample 1 using a multiphoton excitationmicroscope.

FIG. 26 is a view of observing the main surface of the nitridesemiconductor substrate of sample 1 using a multiphoton excitationmicroscope.

FIG. 27 is a view of observing the main surface of the nitridesemiconductor substrate of sample 1 using a multiphoton excitationmicroscope.

FIG. 28 is a view of observing the main surface of the nitridesemiconductor substrate of sample 1 using a multiphoton excitationmicroscope.

FIG. 29 is a view of observing the main surface of the nitridesemiconductor substrate of sample 1 using a multiphoton excitationmicroscope.

FIG. 30 is a view of observing the main surface of the nitridesemiconductor substrate of sample 1 using a multiphoton excitationmicroscope.

FIG. 31 is a view of observing the main surface of the nitridesemiconductor substrate of sample 1 using a multiphoton excitationmicroscope.

FIG. 32 is a view of observing the main surface of the nitridesemiconductor substrate of sample 1 using a multiphoton excitationmicroscope.

FIG. 33 is a view of observing the main surface of the nitridesemiconductor substrate of sample 2 using a multiphoton excitationmicroscope.

DETAILED DESCRIPTION OF THE DISCLOSURE <Finding Obtained by Inventors>

First, findings obtained by inventors will be described.

(i) Dislocation Density

Conventionally, as described above, when the crystal layer is furtherepitaxially grown on the base substrate comprising a single crystal of agroup III nitride semiconductor, for example, a crystal layer on a basesubstrate is grown with only a c-plane as a growth surface withoutexposing inclined interfaces other than the c-plane. In this case, adislocation density in a surface of the crystal layer tended to beinversely proportional to a thickness of the crystal layer.

However, when the crystal layer is grown with only the c-plane as thegrowth surface, the dislocation density in the surface of the crystallayer could not be sufficiently lowered unless the crystal layer isgrown very thick. This causes a reduction of productivity for obtaininga nitride semiconductor substrate having a desired dislocation densityin the main surface.

Accordingly, a technique capable of efficiently obtaining the nitridesemiconductor substrate having a low dislocation density has beendesired.

(ii) Variation in Off-Angle

In the nitride semiconductor substrate, a (0001) plane may be curved ina concave spherical shape with respect to the main surface. When the(0001) plane is curved with respect to the main surface, an off-anglevaries within the main surface, the off-angle being an angle formed by<0001> axis with respect to a normal of the main surface.

The off-angle in the nitride semiconductor substrate affects, forexample, a surface morphology of a semiconductor functional layer grownon the substrate. For example, when a radius of curvature of the (0001)plane of the substrate is small and a variation in the off-angle of thesubstrate is large, the surface morphology of the semiconductorfunctional layer may deteriorate in a part of the substrate, due to theoff-angle. Therefore, when a semiconductor device as a Schottky barrierdiode (SBD) is manufactured using this substrate, a withstand voltageand reliability may decrease in a semiconductor device cut out from aportion where the surface morphology of the semiconductor functionallayer has deteriorated.

Further, for example, when indium (In) is doped on the substrate to forma light emitting layer, the off-angle in the nitride semiconductorsubstrate affects a content of In in the light emitting layer. Forexample, when the radius of curvature of the (0001) plane of thesubstrate is small and the variation in the off-angle of the substrateis large, the content of In in the light emitting layer varies dependingon the variation in the off-angle of the substrate. Therefore, there isa possibility that a light emitting wavelength varies and a lightemitting unevenness occurs in a light emitting element having this lightemitting layer.

Accordingly, a technique capable of reducing the variation in theoff-angle in the nitride semiconductor substrate has been desired, toprevent practical problems such as deterioration of the surfacemorphology and uneven light emission.

The present disclosure is based on the findings of the above (i) and(ii) found by the inventors of the present disclosure.

An Embodiment of the Present Disclosure

Hereinafter, an embodiment of the present disclosure will be describedwith reference to the drawings.

(1) Method for manufacturing a nitride semiconductor substrate

A method for manufacturing a nitride semiconductor substrate accordingto the present embodiment will be described with reference to FIGS. 1 to6.

FIG. 1 is a flowchart illustrating a method for manufacturing a nitridesemiconductor substrate according to the present embodiment. FIGS. 2 (a)to (g), FIGS. 3 (a) to 3 (c), and FIGS. 5 (a) to 6 (b) are schematicsectional views illustrating a part of the method for manufacturing anitride semiconductor substrate according to the present embodiment.FIG. 4 is a schematic perspective view illustrating a part of the methodfor manufacturing a nitride semiconductor substrate according to thepresent embodiment. FIG. 4 corresponds to a perspective view at a timepoint of FIG. 3B, and illustrates a part of a first layer 30 that growson the base substrate 10. Further, in FIG. 5 (b), fine solid lineindicates a crystal plane in the process of growth, and in FIGS. 3 (c)to 6 (b), dotted line indicates a dislocation.

As illustrated in FIG. 1, the method for manufacturing a nitridesemiconductor substrate according to the present embodiment includes,for example, a base substrate preparation step S100, a first step S200,a second step S300, a slicing step S400, and a polishing step S500.

(S100: Base Substrate Preparation Step)

First, in the base substrate preparation step S100, a base substrate 10comprising a single crystal of a group III nitride semiconductor isprepared. In the present embodiment, for example, a gallium nitride(GaN) free-standing substrate is prepared as the base substrate 10.

Hereinafter, in a crystal of a group III nitride semiconductor having awurtzite structure, <0001> axis (for example, [0001] axis) is referredto as “c-axis”, and (0001) plane is referred to as “c-plane”. The (0001)plane may be referred to as a “+c plane (group III element polarplane)”, and the (000-1) plane may be referred to as a “-c plane(nitrogen (N) polar plane)”. Further, <1-100> axis (for example, [1-100]axis) is referred to as “m-axis”, and {1-100} plane is referred to as a“m-plane”. m-axis may be expressed as <10-10> axis. Further, <11-20>axis (for example, [11-20] axis) is referred to as “a-axis”, and {11-20}plane is referred to as “a-plane”.

In the base substrate preparation step S100 of the present embodiment,for example, the base substrate 10 is prepared by a VAS (Void-AssistedSeparation) method.

Specifically, the base substrate preparation step S100 includes: forexample, a substrate preparation step S110 for crystal growth, a firstcrystal layer forming step S120, a metal layer forming step S130, a voidforming step S140, a second crystal layer forming step S150, a peelingstep S160, a slicing step S170, and a polishing step S180.

(S110: Substrate Preparation Step for Crystal Growth)

First, as illustrated in FIG. 2(a), a crystal growth substrate 1(hereinafter, may be abbreviated as a “substrate 1”) is prepared. Thesubstrate 1 is, for example, a sapphire substrate. Also, the substrate 1may be, for example, a Si substrate or a gallium arsenide (GaAs)substrate. The substrate 1 has, for example, a main surface is which isa growth surface. A low index crystal plane closest to the main surface1 s is, for example, a c-plane 1 c.

In the present embodiment, the c-plane 1 c of the substrate 1 isinclined with respect to the main surface 1 s. The c-axis 1 ca of thesubstrate 1 is inclined at a predetermined off-angle θ₀ with respect toa normal of the main surface 1 s. The off-angle θ₀ in the main surface 1s of the substrate 1 is uniform over an entire main surface 1 s. Theoff-angle θ₀ in the main surface is of the substrate 1 affects theoff-angle θ₃ at a center of the main surface 10 s of the base substrate10, which will be described later.

(S120: First Crystal Layer Forming Step)

Next, as illustrated in FIG. 2(b), for example, a low-temperature growthGaN buffer layer and a Si-doped GaN layer are grown in this order as afirst crystal layer (underground growth layer) 2 on the main surface isof the substrate 1, by supplying trimethylgallium (TMG) gas as a groupIII source gas, ammonia gas (NH₃) as a nitrogen source gas, andmonosilane (SiH₄) gas as an n-type dopant gas, to the substrate 1 heatedto a predetermined growth temperature, by a metalorganic vapor phasegrowth (MOVPE) method. At this time, a thickness of the low-temperaturegrowth GaN buffer layer and a thickness of the Si-doped GaN layer are,for example, 20 nm and 0.5 μm, respectively.

(S130: Metal Layer Forming Step)

Next, as illustrated in FIG. 2(c), a metal layer 3 is deposited on thefirst crystal layer 2. The metal layer 3 is, for example, a titanium(Ti) layer. Further, a thickness of the metal layer 3, is for example,20 nm.

(S140: Void Forming Step)

Next, the above-described substrate 1 is put into an electric furnace,and the substrate 1 is placed on a susceptor having a predeterminedheater. After the substrate 1 is placed on the susceptor, the substrate1 is heated by the heater and heat treatment is performed thereto in anatmosphere containing hydrogen gas or hydride gas. Specifically, forexample, the heat treatment is performed at a predetermined temperaturefor 20 minutes in a hydrogen (H₂) gas stream containing 20% NH₃ gas. Aheat treatment temperature is, for example, 850° C. or higher and 1,100°C. or lower. By performing such a heat treatment, the metal layer 3 isnitrided to form a metal nitride layer 5 having high-density fine holeson a surface. Further, by performing the above-described heat treatment,a part of the first crystal layer 2 is etched through the holes of themetal nitride layer 5 to form high-density voids in the first crystallayer 2.

Thereby, as illustrated in FIG. 2(d), a void-containing first crystallayer 4 is formed.

(S150: Second Crystal Layer Forming Step)

Next, for example, a Si-doped GaN layer is epitaxially grown as a secondcrystal layer (full-scale growth layer) 6 over the void-containing firstcrystal layer 4 and metal nitride layer by supplying gallium chloride(GaCl) gas, NH₃ gas and dichlorosilane (SiH₂Cl₂) gas as an n-type dopantgas, to the substrate 1 heated to a predetermined growth temperature bya hydride vapor deposition (HVPE) method. A Ge-doped GaN layer may beepitaxially grown as the second crystal layer 6, by supplyingtetrachlorogerman (GeCl₄) gas or the like instead of SiH₂Cl₂ gas, as ann-type dopant gas.

At this time, the second crystal layer 6 grows from the void-containingfirst crystal layer 4 over the void-containing first crystal layer 4 andthe metal nitride layer 5 through the holes in the metal nitride layer5. A part of the voids in the void-containing first crystal layer 4 isembedded by the second crystal layer 6, but the other part of the voidsin the void-containing first crystal layer 4 remains. A flat cavity isformed between the second crystal layer 6 and the metal nitride layer 5due to the voids remaining in the void-containing first crystal layer 4.This cavity causes peeling of the second crystal layer 6 in a peelingstep S160 described later.

Further, at this time, the second crystal layer 6 is grown by inheritingan orientation of the substrate 1. That is, an off-angle θ₁ in the mainsurface of the second crystal layer 6 is uniform over an entire mainsurface, similarly to an off-angle θ₀ in the main surface is of thesubstrate 1.

Further, at this time, a thickness of the second crystal layer 6 is, forexample, 600 μm or more, preferably 1 mm or more. An upper limit of thethickness of the second crystal layer is not particularly limited, butfrom a viewpoint of improving productivity, the thickness of the secondcrystal layer 6 is preferably 50 mm or less.

(S160: Peeling Step)

After the growth of the second crystal layer 6 is completed, the secondcrystal layer 6 naturally peels off from the substrate 1 at a boundarybetween the void-containing first crystal layer 4 and the metal nitridelayer 5, in a process of cooling a HVPE apparatus used to grow thesecond crystal layer 6.

At this time, tensile stress is introduced into the second crystal layer6 by attracting initial nuclei each other, which are generated in thegrowth process. Therefore, due to the tensile stress generated in thesecond crystal layer 6, an internal stress acts on the second crystallayer 6 so that a surface side thereof is concave. Further, adislocation density in the main surface (front surface) side of thesecond crystal layer 6 is low, while a dislocation density in a backsurface side of the second crystal layer 6 is high. Therefore, even dueto a difference of the dislocation density in a thickness direction ofthe second crystal layer 6, the internal stress acts on the secondcrystal layer 6 so that the surface side thereof is concave.

As a result, as illustrated in FIG. 2(f), after the second crystal layer6 is peeled off from the substrate 1, the surface side thereof is warpedso as to be concave. Therefore, the c-plane 6 c of the second crystallayer 6 is curved in a concave spherical shape with respect to a planeperpendicular to a normal direction of the center of the main surface 6s of the second crystal layer 6. An off-angle θ₂ formed by the c-axis 6ca with respect to the normal of the center of the main surface 6 s ofthe second crystal layer 6 has a predetermined distribution.

(S170: Slicing Step)

Next, as illustrated in FIG. 2 (f), for example, the second crystallayer 6 is sliced by a wire saw along a cut surface SS substantiallyperpendicular to the normal direction of the center of the main surface6 s of the second crystal layer 6.

Thereby, as illustrated in FIG. 2 (g), a base substrate 10 as anas-slice substrate is formed. At this time, a thickness of the basesubstrate 10 is, for example, 450 μm. The off-angle θ₃ of the basesubstrate 10 may change from the off-angle θ₂ of the second crystallayer 6 due to a slice direction dependence.

(S180: Polishing Step)

Next, both sides of the base substrate 10 are polished by a polishingdevice. Thereby, the main surface 10 s of the base substrate 10 ismirror-finished.

By the above-described base substrate preparation step S100, the basesubstrate 10 comprising a single crystal of GaN is obtained.

A diameter of the base substrate 10 is, for example, 2 inches or more. Athickness of the base substrate 10 is, for example, 300 μm or more and 1mm or less.

The main surface 10 s of the base substrate 10 has, for example, a mainsurface (base surface) 10 s which is an epitaxial growth surface. In thepresent embodiment, a lowest index crystal plane closest to the mainplane 10 s is, for example, a c-plane (+c-plane) 10 c.

The c-plane 10 c of the base substrate 10 is curved in a concavespherical shape with respect to the main surface 10 s. The term“spherical” as used herein means a curved surface that is approximatedto a spherical surface. Further, the term “spherical approximation” asused herein means that a sphere is approximated to a perfect circularsphere or an elliptical sphere within a predetermined error.

In the present embodiment, c-plane 10 f of the base substrate 10 has,for example, a curved surface that is approximated to a sphericalsurface in each of a cross section along the m-axis and a cross-sectionalong the a-axis. A radius of curvature of the c-plane 10 c in the basesubstrate 10 is, for example, 1 m or more and less than 10 m.

An off-angle θ₃ formed by the c-axis 10 ca with respect to a normal at acenter of the main surface 10 s of the base substrate 10 has apredetermined distribution.

In the present embodiment, the size of the off-angle θ₃ at the center ofthe main surface 10 s of the base substrate 10 is, for example, 1° orless, preferably 0.4° or less. When the size of the off-angle θ₃ at thecenter of the main surface 10 s exceeds 1°, it may be difficult for thefirst layer 30 to grow three-dimensionally depending on a first growthcondition in the first step S200 described later. Therefore, it becomesdifficult to make the c-plane 30 c disappear. In contrast, according tothe present embodiment, since the size of the off-angle θ₃ at the centerof the main surface 10 s is 1° or less, the first layer 30 can be easilythree-dimensionally grown in the first step S200 described later.Thereby, the c-plane 30 c can easily disappear. Further, since the sizeof the off-angle θ₃ at the center of the main surface 10 s is 0.4° orless, the first layer 30 can grow three-dimensionally under a relativelywide growth condition, and the c-plane 30 c can stably disappear.

From a viewpoint of a three-dimensional growth of the first layer 30,the smaller the size of the off-angle θ₃ at the center of the mainsurface 10 s, the better. However, when the size of the off-angle θ₃ atthe center of the main surface 10 s is too close to 0°, the surface ofthe first layer 30 may be excessively roughened. Therefore, the size ofthe off-angle θ₃ at the center of the main surface 10 s is preferably0.1° or more, for example.

The size and direction of the off-angle θ₃ at the center of the mainsurface 10 s of the base substrate 10 can be adjusted by for example,the size and direction of the off-angle θ₀ of a crystal growth substrate1 used in the above-described VAS method, and the slice angle and slicedirection in the slicing step S170.

Further, according to the present embodiment, for example, the mainsurface 10 s of the base substrate 10 is roughly polished whilemaintaining a so-called epiready state in which a single crystal of agroup III nitride semiconductor can grow epitaxially.

Specifically, the root mean square roughness RMS of the main surface 10s of the base substrate 10 is, for example, 1 nm or more and 10 nm orless. By setting the RMS of the main surface 10 s of the base substrate10 within the above range, it is possible to promote the generation ofthe inclined interface 30 i other than the c-plane on the surface of thefirst layer 30 when the first layer 30 grows on the base substrate 10 inthe first step S200 described later. Further, by setting the RMS of themain surface 10 s of the base substrate 10 within the above range, it ispossible to prevent the surface of the first layer 30 from becomingexcessively rough, and to prevent an average distance L between closesttops described later from becoming shorter in the first layer 30.

Further, according to the present embodiment, for example, a crystalstrain introduced by processing such as the slicing step S170 and thepolishing step S180 of the base substrate 10 may remain on the mainsurface 10 s side of the base substrate 10, while maintaining goodcrystal quality of a bulk portion in the base substrate 10.Specifically, full width at half maximum (FWHM) of a (10-10) planediffraction at the time of performing X-ray locking curve measurementwith an incident angle with respect to the main surface 10 s of the basesubstrate 10 after processing set as 2°, is made larger than a fullwidth at half maximum of the base substrate 10 before processing forexample, and is set as 60 arcsec or more and 200 arcsec or less. Bysetting FWHM of the (10-10) plane diffraction within the above range, itis possible to change a stable crystal plane appearing on the surface ofthe first layer 30 described later due to the crystal strain on the mainsurface 10 s side of the base substrate 10. As a result, the inclinedinterface 30 i other than the c-plane can be generated on the surface ofthe first layer 30. Further, by setting FWHM of the (10-10) planediffraction within the above range, it is possible to prevent excessivedislocations from generating in the first layer 30, which will bedescribed later, due to the crystal strain on the main surface 10 s sideof the base substrate 10.

Further, according to the present embodiment, since the base substrate10 is manufactured by the above-described VAS method, the dislocationdensity on the main surface 10 s of the base substrate 10 is low.Specifically, the dislocation density on the main surface 10 s of thebase substrate 10 is, for example, 3×10⁶ cm⁻² or more and less than1×10⁷ cm⁻².

(S200: First Step (First Layer Growth Step))

After preparing the base substrate 10, as illustrated in FIG. 3 (a), thefollowing first step S200 is performed using the base substrate 10 withno processing performed thereto, that is, neither formation of the masklayer on the main surface 10 s nor formation of a concavo-convex patternis performed on the main surface 10 s. The term “mask layer” as usedherein means, for example, a mask layer used in a so-called ELO(Epitaxial Lateral Overgrown) method and having a predetermined opening.Further, the “concavo-convex pattern” herein means, for example, atleast one of a trench and a ridge used in a so-called pendeoepitaxymethod in which the main surface of the base substrate is directlypatterned. A height difference of the concavo-convex pattern referred toherein is, for example, 100 nm or more. The base substrate 10 of thepresent embodiment is used in the first step S200, in a state of nothaving the above-described structure.

First, as illustrated in FIGS. 3 (b), 3 (c), and FIG. 4, a singlecrystal of a group III nitride semiconductor having a top surface 30 uwith a c-plane 30 c exposed is epitaxially grown directly on the mainsurface 10 s of the base substrate 10. Thereby, a first layer(three-dimensional growth layer) 30 grows.

At this time, a plurality of concaves 30 p formed by being surrounded bythe inclined interface 30 i other than the c-plane are formed on the topsurface 30 u of the single crystal, and the inclined interface 30 i isgradually expanded toward an upper side of the main surface 10 s of thebase substrate 10, and the c-plane 30 c is gradually contracted.Thereby, the c-plane 30 c disappears from the top surface 30 u. As aresult, the first layer 30 whose surface is composed of only theinclined interface 30 i is grown.

That is, in the first step S200, the first layer 30 isthree-dimensionally grown so as to intentionally roughen the mainsurface 10 s of the base substrate 10. Even if the first layer 30 is inan appearance of such a growth form, it is grown as a single crystal asdescribed above. In this regard, the first layer 30 is different from aso-called low temperature growth buffer layer formed as an amorphous orpolycrystal on a dissimilar substrate before epitaxially growing thegroup III nitride semiconductor on the dissimilar substrate such assapphire.

In the present embodiment, for example, a layer comprising the samegroup III nitride semiconductor as the group III nitride semiconductorconstituting the base substrate 10 is epitaxially grown as the firstlayer 30. Specifically, for example, by heating the base substrate 10and supplying GaCl gas and NH₃ gas to the heated base substrate 10 bythe HVPE method, the GaN layer is epitaxially grown as the first layer30.

Here, in the first step S200, in order to express the above-describedgrowth process, for example, the first layer 30 is grown under apredetermined first growth condition.

First, a reference growth condition such that the inclined interface 30i and the c-plane 30 c are neither expanded nor contracted, will bedescribed, with reference to FIG. 7(a). FIG. 7(a) is a schematiccross-sectional view illustrating a growth process under the referencegrowth condition such that the inclined interface and the c-plane areneither expanded nor contracted.

In FIG. 7(a), a thick solid line indicates the surface of the firstlayer 30 for each unit time. The inclined interface 30 i illustrated inFIG. 7(a) is the inclined interface most inclined with respect to thec-plane 30 c. Further, in FIG. 7(a), a growth rate of the c-plane 30 cof the first layer 30 is G_(c0), a growth rate of the inclined interface30 i of the first layer 30 is G_(i), and an angle formed by the c-plane30 c and the inclined interface 30 i in the first layer 30 is α. Also,in FIG. 7(a), the first layer 30 grows while maintaining the angle αformed by the c-plane 30 c and the inclined interface 30 i. Theoff-angle of the c-plane 30 c of the first layer 30 is negligible ascompared with the angle α formed by the c-plane 30 c and the inclinedinterface 30 i.

As illustrated in FIG. 7(a), when each of the inclined interface 30 iand the c-plane 30 c is neither expanded nor contracted, a locus of anintersection between the inclined interface 30 i and the c-plane 30 cbecomes perpendicular to the c-plane 30 c. Therefore, the referencegrowth condition such that each of the inclined interface 30 i and thec-plane 30 c is neither expanded nor contracted, satisfies the followingformula (a).

G _(c0) =G _(i)/cos α  (a)

Next, a first growth condition such that the inclined interface 30 i isexpanded and the c-plane 30 c is contracted, will be described withreference to FIG. 7(b). FIG. 7(b) is a schematic cross-sectional viewillustrating a growth process under the first growth condition such thatthe inclined interface is expanded and the c-plane is contracted.

In FIG. 7(b), as in FIG. 7 (a), a thick solid line indicates the surfaceof the first layer 30 for each unit time. Further, the inclinedinterface 30 i illustrated in FIG. 7(b) is also the inclined interfacemost inclined with respect to the c-plane 30 c. Also, in FIG. 7(b), agrowth rate of the c-plane 30 c of the first layer 30 is G_(c1), and aprogress rate of the locus of the intersection between the inclinedinterface 30 i and the c-plane 30 c of the first layer 30 is R₁.Further, a narrower angle of the angles formed by the c-plane 30 c andthe locus of the intersection between the inclined interface 30 i andthe c-plane 30 c, is am. When the angle formed by R₁ direction and G_(i)direction is α′, α′=α+90−α_(R1) is satisfied. The off-angle of thec-plane 30 c of the first layer 30 is negligible as compared with theangle α formed by the c-plane 30 c and the inclined interface 30 i.

As illustrated in FIG. 7(b), the progress rate R₁ of the locus of theintersection between the inclined interface 30 i and the c-plane 30 c isrepresented by the following formula (b).

R ₁ =G _(i)/cos α′  (b)

Further, a growth rate G_(c1) of the c-plane 30 c of the first layer 30is represented by the following formula (c).

G _(c1) =R ₁ sin α_(R1)  (c)

By substituting the formula (b) into the formula (c), G_(c1) isrepresented by the following formula (d) using G_(i).

G _(c1) =G _(i) sin α_(R1)/cos(α+90−α_(R1))  (d)

In order for the inclined interface 30 i to expand and the c-plane 30 cto contract, α_(R1)<90° is preferable. Accordingly, the first growthcondition such that the inclined interface 30 i is expanded and thec-plane 30 c is contracted, preferably satisfies the following formula(1), due to satisfying formula (d) and α_(R1)<90°.

G _(c1) >G _(i)/cos α  (1)

wherein, as described above, Gi is the growth rate of the interface 30 imost inclined with respect to the c-plane 30 c, and α is the angleformed by the inclined interface 30 i most inclined with respect to thec-plane 30 c, and the c-plane 30 c.

Alternatively, it can be considered that G_(c1) based on the firstgrowth condition is preferably larger than G_(c0) based on the referencegrowth condition. This also derives the formula (1) by substituting theformula (a) into G_(c1)>G_(c0).

Since the growth condition for expanding the inclined interface 30 imost inclined with respect to the c-plane 30 c is a strictest condition,it is possible to expand the other inclined interface 30 i when thefirst growth condition satisfies the formula (1).

Specifically, for example, when the inclined interface 30 i mostinclined with respect to the c-plane 30 c is {10-11} plane, α=61.95° issatisfied. Accordingly, the first growth condition preferably satisfies,for example, the following formula (1′).

G _(c1)>2.13G _(i)  (1′)

Alternatively, as will be described later, for example, when theinclined interface 30 i is {11-2m} plane satisfying m≥3, the inclinedinterface 30 i most inclined with respect to the c-plane 30 c is {11-23}plane, and therefore α=47.3° is satisfied. Accordingly, the first growthcondition preferably satisfies, for example, the following formula (1″).

G _(c1)>1.47G _(i)  (1″)

As the first growth condition of the present embodiment, for example,the growth temperature in the first step S200 is lower than the growthtemperature in the second step S300 described later. Specifically, thegrowth temperature in the first step S200 is, for example, 980° C. orhigher and 1,020° C. or lower, preferably 1,000° C. or higher and 1,020°C. or lower.

Further, as the first growth condition of the present embodiment, forexample, the ratio of a partial pressure of a flow rate of NH₃ gas as anitrogen source gas to a partial pressure of GaCl gas as a group IIIsource gas in the first step S200 (hereinafter, also referred to as“V/III ratio”), may be larger than the V/III ratio in the second stepS300 described later. Specifically, the V/III ratio in the first stepS200 is, for example, 2 or more and 20 or less, preferably 2 or more and15 or less.

Actually, as the first growth condition, at least one of the growthtemperature and the V/III ratio is adjusted within the above range so asto satisfy the formula (1).

Other conditions of the first growth condition according to the presentembodiment are as follows, for example.

Growth pressure: 90 to 105 kPa, preferably 90 to 95 kPaPartial pressure of GaCl gas: 1.5 to 15 kPaN₂ gas flow rate/H₂ gas flow rate: 0 to 1

Here, the first step S200 of the present embodiment is classified intotwo steps based on, for example, a growing form of the first layer 30.Specifically, the first step S200 of the present embodiment includes,for example, an inclined interface expansion step S220 and an inclinedinterface maintenance step S240. By these steps, the first layer 30 has,for example, an expanded inclined interface layer 32 and an inclinedinterface maintaining layer 34.

(S220: Inclined Interface Expansion Step)

First, as illustrated in FIG. 3 (b) and FIG. 4, the expanded inclinedinterface layer 32 of the first layer 30 comprising a single crystal ofa group III nitride semiconductor is epitaxially grown directly on themain surface 10 s of the base substrate 10 under the above-describedfirst growth condition.

In the initial stage of growth of the expanded inclined interface layer32, the expanded inclined interface layer 32 grows in a normal direction(direction along the c-axis) of the main surface 10 s of the basesubstrate 10, with the c-plane 30 c as a growth surface.

By gradually growing the expanded inclined interface layer 32 under thefirst growth condition, as illustrated in FIG. 3(b) and FIG. 4, aplurality of concaves 30 p composed of the inclined interface 30 i otherthan the c-plane, are formed on the top surface 30 u of the expandedinclined interface layer 32 with the c-plane 30 c exposed. The pluralityof concaves 30 p composed of the inclined interface 30 i other than thec-plane are randomly formed on the top surface 30 u. Thereby, theexpanded inclined interface layer 32 is formed, in which the c-plane 30c and the inclined interface 30 i other than the c-plane are mixed onthe surface.

The term “inclined interface 30 i” as used herein means a growthinterface inclined with respect to the c-plane 30 c, and includes lowindex facets other than the c-plane, high-index facets other than thec-plane, or inclined faces that cannot be represented by indices ofcrystal plane (Miller indices). Facets other than the c-plane are, forexample, {11-2m}, {1-10n}, and the like. Wherein, m and n are integersother than 0.

In the present embodiment, since the first growth condition is adjustedso as to satisfy the formula (1) using the above-described basesubstrate 10, for example, {11-2m} plane satisfying m≥3 can be generatedas the inclined interface 30 i. Thereby, an inclination angle of the{11-2m} plane with respect to the c plane 30 c can be loose.Specifically, the inclination angle can be 47.3° or less.

By further growing the expanded inclined interface layer 32 under thefirst growth condition, as illustrated in FIGS. 3(b) and 3 (d), theinclined interface 30 i other than the c-plane is gradually expanded andthe c-plane 30 c is gradually contracted toward the upper side of thebase substrate 10, in the expanded inclined interface layer 32. At thistime, the inclination angle formed by the inclination interface 30 iwith respect to the main surface 10 s of the base substrate 10 graduallydecreases toward the upper side of the base substrate 10. Thereby, mostof the inclined interface 30 i finally becomes the {11-2m} planesatisfying m 3 as described above.

When the expanded inclined interface layer 32 is further grown, thec-plane 30 c of the expanded inclined interface layer 32 disappears fromthe top surface 30 u, and the surface of the expanded inclined interfacelayer 32 is composed only of the inclined interface 30 i. Thereby, amountain-like expanded inclined interface layer 32 is formed in the formof continuous connected cones.

In this way, by forming a plurality of concaves 30 p composed of theinclined interface 30 i other than the c-plane on the top surface 30 uof the expanded inclined interface layer 32, and making the c-plane 30 cdisappear, as illustrated in FIG. 3(c), a plurality of valleys 30 v anda plurality of tops 30 t are formed on the surface of the expandedinclined interface layer 32. Each of the plurality of valleys 30 v isformed as an inflection point that is convex downward on the surface ofthe expanded inclined interface layer 32, and is formed at the upperpart of a position where each of the inclined interfaces 30 i other thanthe c-plane is generated. On the other hand, each of the plurality oftops 30 t is formed as an inflection point that is convex upward on thesurface of the expanded inclined interface layer 32, and is formed at aposition where the c-plane 30 c (finally) disappears and terminates orat the upper part of this position, between a pair of inclinedinterfaces 30 i that expand in opposite directions. The valleys 30 v andthe tops 30 t are formed alternately in a direction along the mainsurface 10 s of the base substrate 10.

According to the present embodiment, in the initial stage of growth ofthe expanded inclined interface layer 32, the expanded inclinedinterface layer 32 is grown to a predetermined thickness on the mainsurface 10 s of the base substrate 10 with the c-plane 30 c as a growthsurface without forming the inclined interface 30 i, and thereafter theinclined interface 30 i other than the c-plane is formed on the surfaceof the expanded inclined interface layer 32. Thereby, a plurality ofvalleys 30 v are formed at positions separated upward from the mainsurface 10 s of the base substrate 10.

Due to the growth process of the expanded inclined interface layer 32 asdescribed above, dislocations are bent and propagated as follows.Specifically, as illustrated in FIG. 3(c), the plurality of dislocationsextending in the direction along the c-axis in the base substrate 10,propagate from the base substrate 10 toward the direction along thec-axis of the expanded inclined interface layer 32. In a region of theexpanded inclined interface layer 32 that has grown with the c-plane 30c as a growth surface, the dislocations propagate in the direction alongthe c-axis of the expanded inclined interface layer 32 from the basesubstrate 10. However, when the dislocations propagated in the directionalong the c-axis of the expanded inclined interface layer 32 are exposedto the inclined interface 30 i, the dislocations are bent and propagatein a direction substantially perpendicular to the inclined interface 30i at a position where the inclined interface 30 i is exposed. That is,the dislocations are bent and propagate in a direction inclined withrespect to the c-axis. Thereby, in the steps after the inclinedinterface expansion step S220, the dislocations are locally collected inthe upper part of the substantially center between the pair of tops 30t. As a result, a dislocation density in the surface of a second layer40, which will be described later, can be lowered.

At this time, in the present embodiment, an average distance between apair of tops separated in a direction along the main surface (alsocalled “an average distance between closest tops”) L is, for example,more than 100 μm, the pair of tops being closest to each other among theplurality of tops, with one of the plurality of valleys sandwichedbetween them, when observing an arbitrary cross section perpendicular tothe main surface 10 s of the base substrate 10. When the averagedistance L between the closest tops is 100 μm or less, as in the casewhere fine hexagonal pyramid-shaped crystal nuclei are generated on themain surface 10 s of the base substrate 10 from the initial stage of theinclined interface expansion step S220, the distance in whichdislocations are bent and propagated is shortened in the steps after theinclined interface expansion step S220. Therefore, the dislocations arenot sufficiently collected in the upper part of the substantially centerbetween the pair of tops 30 t of the expanded inclined interface layer32. As a result, the dislocation density in the surface of the secondlayer 40, which will be described later, may not be sufficientlylowered. In contrast, in the present embodiment, since the averagedistance L between the closest tops is more than 100 μm, at least over50 μm of the distance in which the dislocations are bent and propagatedcan be secured in the steps after the inclined interface expansion stepS220. Thereby, the dislocations can be sufficiently collected in theupper part of the substantially center between the pair of tops 30 t ofthe expanded inclined interface layer 32. As a result, the dislocationdensity in the surface of the second layer 40, which will be describedlater, can be sufficiently lowered.

On the other hand, according to the present embodiment, the averagedistance L between the closest tops is less than 800 μm. When theaverage distance L between the closest tops is 800 μm or more, a heightfrom the valley 30 v to the top 30 t of the expanded inclined interfacelayer 32 on the main surface 10 s of the base substrate 10 becomesexcessively high. Therefore, a thickness of the second layer 40 until itis mirror-finished (it becomes a mirror surface), becomes thicker in thesecond step S300, which will be described later. In contrast, in thepresent embodiment, since the average distance L between the closesttops is less than 800 μm, the height from the valley 30 v to the top 30t of the expanded inclined interface layer 32 on the main surface 10 sof the base substrate 10 can be lowered.

Further, at this time, a first c-plane growth region 60 grown with thec-plane 30 c as a growth surface and an inclined interface growth region70 (gray part in the figure) grown with the inclined interface 30 iother than the c-plane as a growth surface, are formed on the expandedinclined interface layer 320, based on a difference in growth surfacesduring the growth process.

Further, at this time, in the first c-plane growth region 60, a valley60 a is formed at a position where the inclined interface 30 i isgenerated, and a mountain 60 b is formed at a position where the c-plane30 c disappears. Further, in the first c-plane growth region 60, a pairof inclined portions 60 i are formed on both sides of the mountain 60 b,as a locus of an intersection between the c-plane 30 c and the inclinedinterface 30 i.

Further, at this time, when the first growth condition satisfies theformula (1), an angle β formed by the pair of inclined portions 60 i is,for example, 70° or less.

Details of these regions will be described later.

(S240: Inclined Interface Maintenance Step)

After the c-plane 30 c disappears from the surface of the expandedinclined interface layer 32, as illustrated in FIG. 5(a), the growth ofthe first layer 30 over a predetermined thickness is continued whilemaintaining a state where the inclined interface 30 i occupies more thanthe c-plane 30 c on the surface, preferably a state where the surface iscomposed only of the inclined interface 30 i. Thereby, an inclinedinterface maintenance layer 34 having a surface in which the inclinedinterface 30 i occupies more than the c-plane 30 c, is formed on theexpanded inclined interface layer 32. Since the inclined interfacemaintenance layer 34 is formed, the c-plane 30 c can reliably disappearover the entire surface of the first layer 30.

At this time, the c-plane 30 c may reappear in a part of the surface ofthe inclined interface maintenance layer 34, but it is preferable tomainly expose the inclined interface 30 i on the surface of the inclinedinterface maintenance layer 34, so that an area ratio of the inclinedinterface growth region 70 is 80% or more in a creepage cross sectionalong the main surface 10 s of the base substrate 10. The higher thearea ratio occupied by the inclined interface growth region 70 in thecreepage cross section, the better, and it is preferable that the arearatio is 100%.

At this time, the growth condition in the inclined interface maintenancestep S240 is maintained under the above-described first growth conditionin the same manner as in the inclined interface expansion step S220.Thereby, the inclined interface maintenance layer 34 can grow, with onlythe inclined interface 30 i as a growth surface.

Further, at this time, by growing the inclined interface maintenancelayer 34 with the inclined interface 30 i as a growth surface under thefirst growth condition, as described above, the dislocations that bendand propagate in the direction inclined with respect to the c-axis atthe position where the inclined interface 30 i is exposed in theinclined interface expanding layer 32, continue to propagate in the samedirection in the inclined interface maintenance layer 34.

Further, at this time, the inclined interface maintenance layer 34 growswith the inclined interface 30 i as a growth surface, so that the entireinclined interface maintenance layer 34 becomes a part of the inclinedinterface growth region 70.

By the above first step S200, the first layer 30 having the expandedinclined interface layer 32 and the inclined interface maintenance layer34, is formed.

In the first step S200 of the present embodiment, a height from the mainsurface 10 s of the base substrate 10 to the top 30 t of the first layer30 (the maximum height in a thickness direction of the first layer 30)is, for example, more than 100 μm and less than 1.5 mm.

(S300: Second Step (Second Layer Growth Step))

After the first layer 30 in which the c-plane 30 c has disappeared isgrown, a single crystal of a group III nitride semiconductor is furtherepitaxially grown on the first layer 30 as illustrated in FIGS. 5(b) and6(a).

At this time, the inclined interface 40 i is gradually contracted andthe c-plane 40 c is gradually expanded toward the upper side of the mainsurface 10 s of the base substrate 10. Thereby, the inclined interface30 i formed on the surface of the first layer 30 disappears. As aresult, a second layer (flattening layer) 40 having a mirror surface isgrown. The “mirror surface” herein means a surface in which a maximumheight difference of the unevenness of the surface is equal to or lessthan a wavelength of a visible light.

In the present embodiment, for example, a layer is epitaxially grown asthe second layer 40, containing the same group III nitride semiconductoras the group III nitride semiconductor constituting the first layer 30as a main component. In the second step S300, a silicon (Si)-doped GaNlayer is epitaxially grown as the second layer 40, by supplying GaClgas, NH₃ gas and dichlorosilane (SiH₂Cl₂) gas as an n-type dopant gas tothe base substrate 10 heated to a predetermined growth temperature. Asthe n-type dopant gas, GeCl₄ gas or the like may be supplied instead ofthe SiH₂Cl₂ gas.

Here, in the second step S300, in order to express the above-describedgrowth process, for example, the second layer 40 is grown under apredetermined second growth condition.

The second growth condition such that the inclined interface 40 i iscontracted and the c-plane 40 c is expanded, will be described withreference to FIG. 8. FIG. 8 is a schematic cross-sectional viewillustrating a growth process under the second growth condition suchthat the inclined interface is contracted and the c-plane is expanded.FIG. 8 illustrates a process of growing the second layer 40 on the firstlayer 30 where the inclined interface 30 i is exposed, the inclinedinterface 30 i being most inclined with respect to the c-plane 30 c.

In FIG. 8, as in FIG. 7(a), the thick solid line indicates the surfaceof the second layer 40 for each unit time. Further, in FIG. 8, a growthrate of the c-plane 40 c of the second layer 40 is G_(c2), a growth rateof the inclined interface 40 i of the second layer 40 is and a progressrate of the locus of the intersection between the inclined interface 40i and the c-plane 40 c in the second layer 40, is R₂. Further, anarrower angle of the angles formed by the c-plane 30 c and the locus ofthe intersection between the inclined interface 40 i and the c-plane 40c, is α_(R2). When an angle formed by R₂ direction and Gi direction isα″, α″=α−(90−α_(R2)) is satisfied. Further, in FIG. 8, the second layer40 grows while maintaining the angle α formed by the c-plane 30 c andthe inclined interface 30 i in the first layer 30. The off-angle of thec-plane 40 c of the second layer 40 is negligible as compared with theangle α formed by the c-plane 30 c and the inclined interface 30 i.

As illustrated in FIG. 8, the progress rate R₂ of the locus of theintersection between the inclined interface 40 i and the c-plane 40 c,is represented by the following formula (e).

R ₂ =G _(i)/cos α″  (e)

Further, the growth rate G_(c2) of the c-plane 40 c of the second layer40 is represented by the following formula (f).

G _(c2) =R ₂ sin α_(R2)  (f)

By substituting the formula (e) into the formula (f), G_(c2) isrepresented by the following formula (g) using G_(i).

G _(c2) =G _(i) sin α_(R2)/cos(α+α_(R2)−90)  (g)

In order for the inclined interface 40 i to contract and the c-plane 40c to expand, it is preferable that αR₂<90° is satisfied. Accordingly,the second growth condition such that the inclined interface 40 i iscontracted and the c-plane 40 c is expanded, preferably satisfies thefollowing formula (2), du to satisfying the formula (g) and αR₂<90°.

G _(c2) <G _(i)/cos α  (2)

wherein, as described above, G_(i) is the growth rate of the inclinedinterface 40 i most inclined with respect to the c-plane 40 c, and a isthe angle formed by the c-plane 40 c and the inclined interface 40 imost inclined with respect to the c-plane 40 c.

Alternatively, when the growth rate of the c-plane 30 c of the secondlayer 40 under the reference growth condition is G_(c0), it can also beconsidered that G_(c2) under the second growth condition is preferablysmaller than G_(c0) under the reference growth condition. From this aswell, the formula (2) can be derived by substituting the formula (a)into G_(c2)<G_(c0).

When the second growth condition satisfies the formula (2), the otherinclined interface 40 i can also be contracted, because the growthcondition for contracting the interface 40 i most inclined with respectto the c-plane 40 c, is a strictest condition.

Specifically, when the inclined interface 40 i most inclined withrespect to the c-plane 40 c is the {10-11} plane, the second growthcondition preferably satisfies the following formula (2′).

G _(c2)<2.13G _(i)  (2′)

Alternatively, for example, when the inclined interface 30 i is {11-2m}plane satisfying m≥3, the inclined interface 30 i most inclined withrespect to the c-plane 30 c is the {11-23} plane. Therefore, the secondgrowth condition preferably satisfies, for example, the followingformula (2 ″).

G _(c2)<1.47G _(i)  (2″)

As the second growth condition of the present embodiment, the growthtemperature in the second step S300 is set higher than, for example, thegrowth temperature in the first step S200. Specifically, the growthtemperature in the second step S300 is, for example, 990° C. or higherand 1,120° C. or lower, preferably 1,020° C. or higher and 1,100° C. orlower.

Further, as the second growth condition of the present embodiment, theV/III ratio in the second step S300 may be adjusted. For example, theV/III ratio in the second step S300 may be smaller than the V/III ratioin the first step S200. Specifically, the V/III ratio in the second stepS300 is, for example, 1 or more and 10 or less, preferably 1 or more and5 or less.

Actually, as the second growth condition, at least one of the growthtemperature and the V/III ratio is adjusted within the above range so asto satisfy the formula (2).

Other conditions of the second growth condition of the presentembodiment are, for example, as follows.Growth pressure: 90 to 105 kPa, preferably 90 to 95 kPaPartial pressure of GaCl gas: 1.5 to 15 kPaN₂ gas flow rate/H₂ gas flow rate: 1 to 20

Other conditions of the second growth condition of the presentembodiment are, for example, as follows.

Growth pressure: 90 to 105 kPa, preferably 90 to 95 kPaPartial pressure of GaCl gas: 1.5 to 15 kPaN₂ gas flow rate/H₂ gas flow rate: 1 to 20

Here, the second step S300 of the present embodiment is classified intotwo steps based on, for example, a growing form of the second layer 40.Specifically, the second step S300 of the present embodiment includes,for example, a c-plane expansion step S320 and a main growth step S340.By these steps, the second layer 40 has, for example, a c-plane expandedlayer 42 and a main growth layer 44.

(S320: c-Plane Expansion Step)

As illustrated in FIG. 5(b), the c-plane expanded layer 42 of the secondlayer 40 comprising a single crystal of a group III nitridesemiconductor is epitaxially grown on the first layer 30 under theabove-described second growth condition.

At this time, the c-plane 40 c is expanded and the inclined interface 40i other than the c-plane is contracted, toward an upper side of thefirst layer 30.

Specifically, due to the growth under the second growth condition, thec-plane expanded layer 42 grows from the inclined interface 30 i of theinclined interface maintaining layer 34 in a direction perpendicular tothe c-axis (that is, a creepage direction or a lateral direction) withthe inclined interface 40 i as a growth surface. When the c-planeexpanded layer 42 is grown laterally, the c-plane 40 c of the c-planeexpanded layer 42 begins to be exposed again in the upper part of thetop 30 t of the inclined interface maintenance layer 34. Thereby, thec-plane expanded layer 42 is formed, in which the c-plane 40 c and theinclined interface 40 i other than the c-plane are mixed on the surface.

When the c-plane expanded layer 42 is further grown laterally, thec-plane 40 c gradually expands, and the inclined interface 40 i of thec-plane expanded layer 42 gradually contracts. Thereby, the concaves 30p formed by the plurality of inclined interfaces 30 i are graduallyembedded in the surface of the first layer 30.

Thereafter, when the c-plane expanded layer 42 is further grown, theinclined interface 40 i of the c-plane expanded layer 42 disappearscompletely, and the concaves 30 p composed of the plurality of inclinedinterfaces 30 i on the surface of the first layer 30 are completelyembedded. Thereby, the surface of the c-plane expanded layer 42 becomesa mirror surface (flat surface) composed only of the c-plane 40 c.

At this time, the dislocation density can be lowered by locallycollecting dislocations during the growth process of the first layer 30and the c-plane expanded layer 42. Specifically, the dislocations thatbend and propagate in the direction inclined with respect to the c-axisin the first layer 30 continue to propagate in the same direction in thec-plane expanded layer 42. Thereby, the dislocations are collectedlocally at a meeting part of the adjacent inclined interfaces 40 i inthe upper part of the center of the c-plane expanded layer 42 betweenthe pair of tops 30 t. Of the plurality of dislocations collected at themeeting part of the adjacent inclined interface 40 i of the c-planeexpanded layer 42, the dislocations having Burgers vectors opposite toeach other, disappear at the meeting. Further, some of the dislocationscollected at the meeting part of the adjacent inclined interfaces 40 iform a loop, and the propagation in the direction along the c-axis (thatis, toward the surface side of the c-plane expanded layer 42) issuppressed. The other part of the plurality of dislocations collected atthe meeting part of the adjacent inclined interfaces 40 i of the c-planeexpanded layer 42, changes its propagation direction again from thedirection inclined with respect to the c-axis to the direction along thec-axis, and propagates to the surface side of the second layer 40. Inthis way, by making some of the plurality of dislocations disappear andsuppressing the propagation of some of the plurality of dislocations tothe surface side of the c-plane expanded layer 42, the dislocationdensity in the surface of the second layer 40 can be lowered. Further,by collecting the dislocations locally, a low dislocation density regioncan be formed in the upper side of a portion of the second layer 40 inwhich the dislocations propagate in the direction inclined with respectto the c-axis.

Further, at this time, since the c-plane 40 c gradually expands in thec-plane expanded layer 42, a second c-plane growth region 80 that hasgrown with the c-plane 40 c as a growth surface, which will be describedlater, is formed while gradually expanding toward the upper side in thethickness direction.

On the other hand, in the c-plane expanded layer 42, as the inclinedinterface 40 i gradually contracts, the inclined interface growth region70 gradually contracts toward the upper side in the thickness direction,and terminates at a predetermined position in the thickness direction.Due to the growth process of the c-plane expanded layer 42 as describedabove, the valley 70 a of the inclined interface growth region 70 isformed at a position where the c-plane 40 c is generated again, in across-sectional view. Further, in the process of gradually embedding theconcave formed by the inclined interface 40 i, a mountain 70 b of theinclined interface growth region 70 is formed at a position where theinclined interface 40 i disappears, in a cross-sectional view.

In the c-plane expansion step S320, the surface of the c-plane expandedlayer 42 is a mirror surface composed only of the c-plane 40 c, andtherefore the height of the c-plane expanded layer 42 in the thicknessdirection (maximum height in the thickness direction) is, for example,greater than or equal to the height from the valley 30 v to the top 30 tof the inclined interface maintenance layer 34.

(S340: Main Growth Step (c-Plane Growth Step))

When the inclined interface 40 i disappears in the c-plane expandedlayer 42 and the surface is mirror-finished, as illustrated in FIG.6(a), a main growth layer 44 is formed on the c-plane expanded layer 42over a predetermined thickness with the c-plane 40 c as a growthsurface. Thereby, the main growth layer 44 having only the c-plane 40 con the surface without having the inclined interface 40 i, is formed.

At this time, the growth condition in the main growth step S340 ismaintained under the above-described second growth condition, in thesame manner as in the c-plane expansion step S320. Thereby, the maingrowth layer 44 can be step-flow-grown with the c-plane 40 c as a growthsurface.

Further, at this time, a radius of curvature of the c-plane 40 c of thegrowth layer 44 can be larger than a radius of curvature of the c-plane10 c of the base substrate 10. Thereby, a variation in the off-angle ofthe c-axis with respect to the normal of the surface of the main growthlayer 44 can be smaller than the variation in the off-angle of thec-axis 10 ca with respect to the normal of the main surface 10 s of thebase substrate 10.

Further, at this time, by growing the main growth layer 44 with only thec-plane 40 c as a growth surface without exposing the inclined interface40 i, a second c-plane growth region 80, which will be described later,is formed over an entire growth layer 44.

In the main growth step S340, a thickness of the main growth layer 44is, for example, 300 μm or more and 10 mm or less. Since the thicknessof the main growth layer 44 is 300 μm or more, at least one or moresubstrates 50 can be sliced from the main growth layer 44 in the slicingstep S400 described later. On the other hand, since the thickness of themain growth layer 44 is 10 mm, at least ten substrates 50 can beobtained when a final thickness is 650 and 700 μm-thick substrate 50 issliced from the main growth layer 44, even if the karfloss of about 200μm is taken into consideration.

By the above second step S300, the second layer 40 having the c-planeexpanded layer 42 and the main growth layer 44 is formed. As a result, alaminated structure 90 of the present embodiment is formed.

The above steps from the first step S200 to the second step S300, arecontinuously performed in the same chamber without exposing the basesubstrate 10 to the atmosphere. Thereby, it is possible to suppress aformation of an unintended high oxygen concentration region (a regionhaving an oxygen concentration excessively higher than the inclinedinterface growth region 70), at an interface between the first layer 30and the second layer 40.

(S400: Slicing Step)

Next, as illustrated in FIG. 6(b), for example, the main growth layer 44is sliced by a wire saw along a cut surface substantially parallel tothe surface of the main growth layer 44. Thereby, at least one nitridesemiconductor substrate 50 (also referred to as a substrate 50) as anas-sliced substrate is formed. At this time, the thickness of thesubstrate 50 is, for example, 300 μm or more and 700 μm or less.

At this time, the radius of curvature of the c-plane 50 c of thesubstrate 50 can be larger than the radius of curvature of the c-plane10 c of the substrate 10. Also, at this time, the radius of curvature ofthe c-plane 50 c of the substrate 50 can be larger than the radius ofcurvature of the c-plane 40 c of the main growth layer 44 beforeslicing. Thereby, the variation in the off-angle θ of the c-axis 50 cawith respect to the normal of the main surface 50 s of the substrate 50can be smaller than the variation in the off-angle of the c-axis 10 caof the substrate 10.

(S500: Polishing Step)

Next, both sides of the substrate 50 are polished by a polishing device.At this time, the thickness of the final substrate 50 is, for example,250 μm or more and 650 μm or less.

The substrate 50 according to the present embodiment is manufactured bythe above steps S100 to S500.

(A Step of Preparing a Semiconductor Laminate and a Step of Preparing aSemiconductor Device)

After the substrate 50 is manufactured, for example, a semiconductorfunctional layer including a group III nitride semiconductor isepitaxially grown on the substrate 50 to prepare a semiconductorlaminate. After the semiconductor laminate is prepared, an electrode orthe like is formed using the semiconductor laminate, and thesemiconductor laminate is diced, and a chip having a predetermined sizeis cut out. Thereby, a semiconductor device is prepared.

(2) Laminated Structure

Next, a laminated structure 90 according to the present embodiment willbe described with reference to FIG. 6(a).

The laminated structure 90 of the present embodiment has, for example, abase substrate 10, a first layer 30, and a second layer 40.

The first layer 30 grows on, for example, the main surface 10 s of thebase substrate 10.

The first layer 30 has, for example, a plurality of valleys 30 v and aplurality of tops 30 t which are formed by forming a plurality ofconcaves 30 p composed of the inclined interfaces 30 i other than thec-plane and making the c-plane 30 c disappear. When observing anarbitrary cross section perpendicular to the main surface of the basesubstrate 10, an average distance between the closest tops is, forexample, more than 100 μm.

Further, the first layer 30 includes, for example, a first c-planegrowth region (first low oxygen concentration region) 60 and an inclinedinterface growth region (high oxygen concentration region) 70 based on adifference of the growth surface in the growth process.

The first c-plane growth region 60 is a region that has grown with thec-plane 30 c as a growth surface. The first c-plane growth region 60has, for example, a plurality of valleys 60 a and a plurality ofmountains 60 b in a cross-sectional view. Each of the valleys 60 a andthe mountains 60 b referred to herein, means a part of a shape observedbased on the difference of emission intensity when the cross section ofthe laminated structure 90 is observed using a fluorescence microscopeor the like, and does not mean a part of an outermost surface shapegenerated during the growth of the first layer 30. Each of the pluralityof valleys 60 a is an inflection point that is convex downward in thefirst c-plane growth region 60 in a cross-sectional view, and is formedat a position where the inclined interface 30 i is generated. At leastone of the plurality of valleys 60 a is provided at a position separatedupward from the main surface 10 s of the base substrate 10. On the otherhand, each of the plurality of mountains 60 b is an inflection pointthat is convex upward in the first c-plane growth region 60 in across-sectional view, and is formed at a position where the c-plane 30 cdisappears (finally) and terminates, between a pair of inclinedinterfaces 30 i that expand in opposite directions. The valleys 60 a andthe mountains 60 b are formed alternately in a direction along the mainsurface 10 s of the base substrate 10.

When observing an arbitrary cross section perpendicular to the mainsurface 10 s of the base substrate 10, an average distance between thepair of mountains 60 b separated in the direction along the main surface10 s of the substrate 10 corresponds to an average distance L betweenthe closest tops of the first layer 30 described above, and is, forexample, more than 100 μm, the pair of mountains 60 b being closest toeach other among the plurality of mountains 60 b, with one of theplurality of valleys 60 a sandwiched between them.

The first c-plane growth region 60 has a pair of inclined portions 60 iprovided as a locus of an intersection between the c-plane 30 c and theinclined interface 30 i, on both sides interposing one of the pluralityof mountains 60 b. The inclined portion 60 i referred to herein means apart of the shape observed based on the difference of emission intensitywhen the cross section of the laminated structure 90 is observed using afluorescence microscope or the like, and does not mean the inclinedinterface 30 i on the outermost surface that generates during the growthof the first layer 30.

An angle β formed by the pair of inclined portions 60 i is, for example,70° or less, preferably 20° or more and 65° or less, in across-sectional view.

The above matter: the angle β formed by the pair of inclined portions 60i is 70° or less, means that the ratio G_(c1)/G_(i) is high, which isthe ratio of the growth rate G_(c1) of the c-plane 30 c of the firstlayer 30 to the growth rate G_(i) of the inclined interface 30 i mostinclined with respect to the c-plane 30 c of the first layer 30.Thereby, the inclined interface 30 i other than the c-plane can beeasily generated. As a result, the dislocations can be easily bent at aposition where the inclined interface 30 i is exposed. Further, sincethe angle β formed by the pair of inclined portions 60 i is 70° or less,a plurality of valleys 30 v and a plurality of tops 30 t can be easilygenerated in the upper part of the main surface 10 s of the basesubstrate 10. Further, since the angle β formed by the pair of inclinedportions 60 i is 65° or less, the inclined interface 30 i other than thec-plane can be more easily generated, and a plurality of valleys 30 vand a plurality of tops 30 t can be more easily generated in the upperpart of the main surface 10 s of the base substrate 10. Also, since theangle β formed by the pair of inclined portions 60 i is 20° or more, theheight from the valley 30 v to the top 30 t of the first layer 30 isprevented from increasing, and the thickness until the second layer 40is mirror-finished, is prevented from increasing.

On the other hand, the inclined interface growth region 70 is a regiongrown with the inclined interface 30 i other than the c-plane as agrowth surface. A lower surface of the inclined interface growth region70 is formed, for example, following the shape of the first c-planegrowth region 60. The inclined interface growth region 70 iscontinuously provided along the main surface of the base substrate 10.

In the inclined interface growth region 70, oxygen is easily taken in ascompared with the first c-plane growth region 60. Therefore, the oxygenconcentration in the inclined interface growth region 70 is higher thanthe oxygen concentration in the first c-plane growth region 60. Theoxygen taken into the inclined interface growth region 70, is, forexample, the oxygen unintentionally mixed in a vapor phase growthapparatus, or the oxygen released from a member (quartz member or thelike) constituting the vapor phase growth apparatus, or the like.

The oxygen concentration in the first c-plane growth region 60 is, forexample, 5×10¹⁶ cm⁻³ or less, preferably 3×10¹⁶ cm⁻³ or less. On theother hand, the oxygen concentration in the inclined interface growthregion 70 is, for example, 9×10¹⁷ cm⁻³ or more and 5×10¹⁹ cm⁻³ or less.

The second layer 40 has, for example, the inclined interface growthregion (high oxygen concentration region) 70 and the second c-planegrowth region (second low oxygen concentration region) 80 based on adifference of the growth surface in the growth process.

The top surface of the inclined interface growth region 70 in the secondlayer 40 has, for example, a plurality of valleys 70 a and a pluralityof mountains 70 b in a cross-sectional view. Each of the valleys 70 aand the mountains 70 b referred to herein, means a part of the shapeobserved based on the difference of emission intensity when the crosssection of the laminated structure 90 is observed using a fluorescencemicroscope or the like, and does not mean a part of the outermostsurface shape that generates during the growth of the second layer 40.As described above, the plurality of valleys 70 a of the inclinedinterface growth region 70 are formed at positions where the c-plane 40c is generated again, in the cross-sectional view. Further, theplurality of valleys 70 a of the inclined interface growth region 70 areformed at the upper part of the plurality of mountains 60 b of the firstc-plane growth region 60, respectively, in a cross-sectional view. Onthe other hand, as described above, the plurality of mountains 70 b ofthe inclined interface growth region 70 are formed respectively at aposition where the inclined interface 40 i disappears and terminates, ina cross-sectional view. Further, the plurality of mountains 70 b of theinclined interface growth region 70 are formed at the upper part of theplurality of valleys 60 a of the first c-plane growth region 60,respectively, in a cross-sectional view.

Further, a surface of the second layer 40, which is substantiallyparallel to the main surface 10 s of the base substrate 10 at an upperend of the inclined interface growth region 70 is formed as a boundarysurface 40 b at a position where the inclined interface 40 i of thesecond layer 40 disappears and terminates.

The second c-plane growth region 80 is a region that has grown with thec-plane 40 c as a growth surface. In the second c-plane growth region80, oxygen uptake is suppressed as compared with the inclined interfacegrowth region 70. Therefore, the oxygen concentration in the secondc-plane growth region 80 is lower than the oxygen concentration in theinclined interface growth region 70. The oxygen concentration in thesecond c-plane growth region 80 is, for example, 5×10¹⁶ cm⁻³ or less,preferably 3×10¹⁶ cm⁻³ or less.

In the present embodiment, in the growth process of the first layer 30,the dislocations bend and propagate in a direction substantiallyperpendicular to the inclined interface 30 i at a position where theinclined interface 30 i other than the c-plane is exposed. Thereby, inthe second layer 40, some of the plurality of dislocations disappear,and some of the plurality of dislocations are suppressed frompropagating to the surface side of the c-plane expanded layer 42.Thereby, the dislocation density in the surface of the second layer 40is lower than the dislocation density in the main surface 10 s of thebase substrate 10.

Further, in the present embodiment, the dislocation density in thesurface of the second layer 40 is sharply reduced in a thicknessdirection.

Here, the dislocation density in the main surface 10 s of the basesubstrate 10 is N₀, and the dislocation density in the boundary surface40 b at the position where the inclined interface 40 i disappears in thesecond layer 40, is N. Also, an average dislocation density in theboundary surface 40 b is N. On the other hand, when the crystal layer ofa group III nitride semiconductor is epitaxially grown on the mainsurface 10 s of the base substrate 10 to a thickness equal to thethickness from the main surface to the boundary surface 40 b of the basesubstrate 10 of the present embodiment, with only the c-plane as agrowth surface (hereinafter, it is also referred to as “in the case ofc-plane limited growth”), the dislocation density in the surface of thecrystal layer is N′.

In the case of the c-plane limited growth, the dislocation density inthe surface of the crystal layer tended to be inversely proportional tothe thickness of the crystal layer. Specifically, in the case of thec-plane limited growth, when the thickness of the crystal layer is 1.5mm, a reduction rate of the dislocation density obtained by N′/N₀ isabout 0.6.

In contrast, in the present embodiment, the reduction rate of thedislocation density obtained by N/N₀ is smaller than, for example, thereduction rate of the dislocation density obtained by N′/N₀ in the caseof the c-plane limited growth.

Specifically, in the present embodiment, the thickness of the boundarysurface 40 b at the position where the inclined interface 40 idisappears in the second layer 40 from the main surface 10 s of the basesubstrate 10 is, for example, 1.5 mm or less, preferably 1.2 mm or less.Further, in the present embodiment, the reduction rate of thedislocation density obtained by N/N₀ described above is, for example,0.3 or less, preferably 0.23 or less, and more preferably 0.15 or less.

In the present embodiment, a lower limit of the thickness of the basesubstrate 10 from the main surface 10 s to the boundary surface 40 b isnot limited, because the thinner, the better. However, in the first stepS200 and the second step S300, the thickness of the base substrate 10from the main surface 10 s to the boundary surface 40 b is, for example,more than 200 μm, in consideration of the process from the generation ofthe inclined interface 30 i to the disappearance of the inclinedinterface 40 i.

Further, in the present embodiment, a lower limit of the reduction rateof the dislocation density is not limited, because the smaller, thebetter. However, the reduction rate of the dislocation density is, forexample, 0.01 or more, in consideration of the matter that the thicknessfrom the main surface 10 s of the base substrate 10 to the boundarysurface 40 b is 1.5 mm or less.

In addition, in the present embodiment, an entire surface of the secondlayer 40 is composed of +c plane, and the first layer 30 and the secondlayer 40 do not include a polarity reversal zone (inversion domain),respectively. In this respect, the laminated structure 90 of the presentembodiment is different from a laminated structure formed by a so-calledDEEP (Dislocation Elimination by the Epitaxial-growth withinverse-pyramidal Pits) method, that is, different from a laminatedstructure including the polarity reversal zone in a core located in thecenter of a pit.

(3) Nitride Semiconductor Substrate (Nitride Semiconductor Free-StandingSubstrate, Nitride Crystal Substrate)

Next, a nitride semiconductor substrate 50 according to the presentembodiment will be described with reference to FIG. 9. FIG. 9(a) is aschematic top view illustrating a nitride semiconductor substrateaccording to the present embodiment, (b) is a schematic cross-sectionalview taken along the m-axis of the nitride semiconductor substrateaccording to the present embodiment, and (c) is a schematiccross-sectional view taken along the a-axis of the nitride semiconductorsubstrate according to the present embodiment. A direction along them-axis is an x-direction, and a direction along the a-axis is ay-direction.

In the present embodiment, the substrate 50 obtained by slicing thesecond layer 40 by the above-described manufacturing method is, forexample, a free-standing substrate comprising a single crystal of agroup III nitride semiconductor. In the present embodiment, thesubstrate 50 is, for example, a GaN free-standing substrate.

A diameter of the substrate 50 is, for example, 2 inches or more. Athickness of the substrate 50 is, for example, 300 μm or more and 1 mmor less.

A conductivity of the substrate 50 is not particularly limited, but whenmanufacturing a semiconductor device as a vertical Schottky barrierdiode (SBD) using the substrate 50, the substrate 50 is, for example,n-type, and n-type impurities in the substrate 50 are, for example, Sior germanium (Ge), and n-type impurities concentration in the substrate50 is, for example, 1.0×10¹⁸ cm⁻³ or more and 1.0×10²⁰ cm⁻³ or less.

The substrate 50 has, for example, a main surface 50 s which is anepitaxial growth surface. In the present embodiment, a lowest indexcrystal plane closest to the main surface 50 s is, for example, thec-plane 50 c.

The main surface 50 s of the substrate 50 is mirror-finished, forexample, and a root mean square roughness RMS of the main surface 50 sof the substrate 50 is, for example, less than 1 nm.

Further, in the present embodiment, the impurity concentration in thesubstrate 50 obtained by the above-described manufacturing method islower than that of the substrate obtained by a flux method, anammonothermal method, or the like.

Specifically, a hydrogen concentration in the substrate 50 is, forexample, less than 1×10¹⁷ cm⁻³, preferably 5×10¹⁶ cm⁻³ or less.

Further, in the present embodiment, the substrate 50 is formed byslicing the main growth layer 44 grown with the c-plane 40 c as a growthsurface, and therefore does not include the inclined interface growthregion 70 grown with the inclined interface 30 i or the inclinedinterface 40 i as a growth surface. That is, the entire body of thesubstrate 50 is configured by a low oxygen concentration region.

Specifically, an oxygen concentration in the substrate 50 is, forexample, 5×10¹⁶ cm⁻³ or less, preferably 3×10¹⁶ cm⁻³ or less.

Further, in the present embodiment, the substrate 50 does not include,for example, the polarity reversal zone (inversion domain) as describedabove.

(Curvature of c-Plane and Variation in Off-Angle)

As illustrated in FIGS. 9(b) and 9 (c), in the present embodiment, thec-plane 50 c as a lowest index crystal plane closest to the main surface50 s of the substrate 50 is curved in a concave spherical shape withrespect to the main surface 50 s, due to, for example, theabove-described method for manufacturing the substrate 50.

In the present embodiment, the c-plane 50 c of the substrate 50 has, forexample, a curved surface that approximates a spherical surface in eachof a cross section along the m-axis and a cross-section along thea-axis.

In the present embodiment, since the c-plane 50 f of the substrate 50 iscurved like a concave spherical surface as described above, at least apart of the c-axis 50 ca is inclined with respect to the normal of themain surface 50 s. The off-angle θ, which is the angle formed by thec-axis 50 ca with respect to the normal of the main surface 50 s, has apredetermined distribution within the main surface 50 s.

In the off-angle θ formed by the c-axis 50 ca with respect to the normalof the main surface 50 s, a directional component along the m-axis is“θ_(m)”, and a directional component along the a-axis is “θ_(a)”, andθ²=θ_(m) ²+θ_(a) ² is satisfied.

In the present embodiment, since the c-plane 50 c of the substrate 50 iscurved like a concave spherical surface as described above, theoff-angle m-axis component θ_(m) and the off-angle α-axis componentθ_(a) can be approximately represented by a linear function of x and alinear function of y, respectively.

In the present embodiment, a radius of curvature of the c-plane 50 c ofthe substrate 50 is larger than, for example, a radius of curvature ofthe c-plane 10 c in the base substrate 10 which is used in theabove-described method for manufacturing the substrate 50.

Specifically, the radius of curvature of the c-plane 50 c of thesubstrate 50 is, for example, 23 m or more, preferably 30 m or more, andmore preferably 40 m or more.

For a reference, in the case of the c-plane limited growth, the radiusof curvature of the c-plane of the substrate sliced from the crystallayer having the same thickness as a total thickness of the first layer30 and the second layer 40 of the present embodiment may be larger thanthe radius of curvature of the c-plane 10 c in the base substrate 10.However, in the case of the c-plane limited growth, the radius ofcurvature of the c-plane of the substrate sliced from the crystal layeris about 11 m when the thickness of the crystal layer is 2 mm, and isabout 1.4 times the radius of curvature of the c-plane 10 c in the basesubstrate 10.

In the present embodiment, an upper limit of the radius of curvature ofthe c-plane 50 c of the substrate 50 is not particularly limited,because the larger, the better. When the c-plane 50 c of the substrate50 is substantially flat, it may be considered that the radius ofcurvature of the c-plane 50 c is infinite.

Further, in the present embodiment, since the radius of curvature of thec-plane 50 c of the substrate 50 is large, the variation in theoff-angle θ of the c-axis 50 ca with respect to the normal of the mainsurface 50 s of the substrate 50 can be smaller than the variation inthe off-angle of the c-axis 10 ca of the substrate 10.

Specifically, when the X-ray locking curve of the (0002) plane of thesubstrate 50 is measured and the off-angle θ of the c-axis 50 ca withrespect to the normal of the main plane 50 s is measured based on adiffraction peak angle of the (0002) plane, the variation obtained bythe difference between maximum and minimum in the size of the off-angleθ within a diameter of 29.6 mm from the center of the main surface 50 sis, for example, 0.075° or less, preferably 0.057° or less, and morepreferably 0.043° or less.

For a reference, in the base substrate 10 prepared by theabove-described VAS method, the variation in the off-angle of the c-axis10 ca obtained by the above-described measurement method is about 0.22°.Further, in the case of the c-plane limited growth, when a thickness ofthe crystal layer is the same as a total thickness of the first layer 30and the second layer 40 of the present embodiment (for example, 2 mm),the variation in the off-angle of the c-axis obtained by theabove-described measuring method is about 0.15° in the nitridesemiconductor substrate obtained from the crystal layer.

In the present embodiment, a lower limit of the variation in theoff-angle θ of the c-axis 50 ca of the substrate 50 is not particularlylimited, because the smaller, the better. When the c-plane 50 c of thesubstrate 50 is substantially flat, it may be considered that thevariation in the off-angle θ of the c-axis 50 ca of the substrate 50 is0°.

Further, in the present embodiment, since the curvature of the c-plane50 c is isotropically small with respect to the main surface 50 s of thesubstrate 50, the radius of curvature of the c-plane 50 c has littledirection dependence.

Specifically, the difference between the radius of curvature of thec-plane 50 c in the direction along the a-axis and the radius ofcurvature of the c-plane 50 c in the direction along the m-axis obtainedby the above-described measurement method is, for example, 50% or less,preferably 20% or less of the larger radius of curvature.

(Dark Spot)

Next, a dark spot on the main surface 50 s of the substrate 50 of thepresent embodiment will be described. The “dark spot” referred to hereinmeans a point where an emission intensity is low in an observation imageof the main surface 50 s observed using a multiphoton excitationmicroscope, or a cathode luminescence image of the main surface 50 s,and includes not only dislocations but also non-emissive centers due toforeign matters or point defects. The “multiphoton excitationmicroscope” is sometimes referred to as a multiphoton excitationfluorescence microscope.

In the present embodiment, since the substrate 50 is manufactured usingthe base substrate 10 comprising a high-purity GaN single crystalprepared by the VAS method, there are few non-emission centers in thesubstrate 50 due to foreign matters or point defects. Accordingly, whenthe main surface of the substrate 50 is observed using a multiphotonexcitation microscope or the like, 95% or more, preferably 99% or moreof the dark spots are dislocations rather than non-emission centers dueto foreign matters or point defects.

Further, in the present embodiment, by the above-described manufacturingmethod, the dislocation density in the surface of the second layer 40 islower than the dislocation density in the main surface 10 s of the basesubstrate 10. Thereby, the dislocations are also lowered in the mainsurface 50 s of the substrate 50 formed by slicing the second layer 40.

Further, in the present embodiment, since the first step S200 and thesecond step S300 are performed by the above-described manufacturingmethod using the base substrate 10 in an unprocessed state, regions withhigh dislocation density due to the concentration of dislocations arenot formed, and regions with low dislocation density are uniformlyformed on the main surface 50 s of the substrate 50 formed by slicingthe second layer 40.

Specifically, in the present embodiment, observation of the main surface50 s of the substrate 50 using the multiphoton excitation microscope ina field of view of 250 μm square to obtain a dislocation density from adark spot density, reveals that there is no region where the dislocationdensity exceeds 3×10⁶ cm⁻², and a region having a dislocation density ofless than 1×10⁶ cm⁻² exists in 80% or more, preferably 90% or more, morepreferably 95% or more of the main surface 50 s.

In the case of using the manufacturing method of the present embodiment,an upper limit value of a proportion of the region where the dislocationdensity is less than 1×10⁶ cm⁻² is preferably close to 100%, but may be,for example, 99% of the main surface 50 s.

Further, in the present embodiment, the dislocation density obtained byaveraging an entire main surface 50 s of the substrate 50 is, forexample, less than 1×10⁶ cm⁻², preferably less than 5.5×10⁵ cm⁻² andmore preferably 3×10⁵ cm⁻² or less.

Further, the main surface 50 s of the substrate 50 of the presentembodiment includes, for example, a dislocation-free region of at least50 μm square based on an average distance L between the closest tops inthe first step S200 described above. Further, 50 μm squaredislocation-free regions are scattered over the entire main surface 50 sof the substrate 50, for example. Further, the main surface 50 s of thesubstrate 50 of the present embodiment includes, for example, 50 μmsquare dislocation-free regions that do not overlap at a density of100/cm² or more, preferably 800/cm² or more, and more preferably1600/cm² or more. When the density of the 50 μm square dislocation-freeregions that do not overlap is 1600/cm² or more, for example, itcorresponds to a case where the main surface 50 s includes at least one50 μm square dislocation-free regions in an arbitrary field of view of250 μm square.

An upper limit of the density of the 50 μm square dislocation-freeregions that do not overlap is 40,000/cm² based on the measurementmethod.

For a reference, in a substrate obtained by a conventional manufacturingmethod that does not perform a special process of collectingdislocations, the size of the dislocation-free region is smaller than 50μm square, or the density of the 50 μm square dislocation-free region islower than 100/cm².

Next, Burgers vector of the dislocations in the substrate 50 of thepresent embodiment will be described.

In the present embodiment, since the dislocation density in the mainsurface 10 s of the base substrate 10 used in the above-describedmanufacturing method is low, a plurality of dislocations are less likelyto be combined (mixed) when the first layer 30 and the second layer 40are grown on the base substrate 10. This makes it possible to suppressthe formation of dislocations having a large Burgers vector in thesubstrate 50 obtained from the second layer 40.

Specifically, in the substrate 50 of the present embodiment, forexample, there are many dislocations whose Burgers vector is either<11-20>/3, <0001>, or <11-23>/3. The “Burgers vector” herein can bemeasured by, for example, a large-angle convergent-beam electrondiffraction method (LACBED method) using a transmission electronmicroscope (TEM). Further, dislocations whose Burgers vector is<11-20>/3 are edge dislocations, and dislocations whose Burgers vectoris <0001> are screw dislocations, and dislocations whose Burgers vectoris <11-23>/3 are mixed dislocations in which edge dislocations and screwdislocations are mixed.

In the present embodiment, random extraction of 100 dislocations on themain surface 50 s of the substrate 50 reveals that a percentage ofdislocations whose Burgers vector is either <11-20>/3, <0001> or<11-23>/3, is, for example, 50% or more, preferably 70% or more, andmore preferably 90% or more. Dislocations whose Burgers vector is2<11-20>/3 or <11-20> may be present in at least a part of the mainsurface 50 s of the substrate 50.

(Regarding X-Ray Locking Curve Measurement with a Different Slit Width)

Here, the inventors found that by measuring the X-ray locking curve witha different slit width on an incident side, both the crystal qualityfactor constituting the substrate 50 of the present embodiment and thecurvature (warp) of the c-plane 50 c described above can be evaluated atthe same time.

First, an influence of a crystal quality factor on the X-ray lockingcurve measurement will be described.

A full width at half maximum of a diffraction pattern in the X-raylocking curve measurement is greatly affected by crystal quality factorssuch as high/low dislocation density, high/low mosaicity, high/lowstacking fault density, high/low basal plane dislocation density,high/low point defect density (vacancy, etc.), large or small amount ofin-plane fluctuation of lattice constant, and a distribution of animpurity concentration. When these crystal quality factors are not good,a fluctuation of a diffraction angle in the X-ray locking curvemeasurement becomes large, and the full width at half maximum of thediffraction pattern becomes large.

Next, an influence of the curvature of the c-plane 50 c in the X-raylocking curve measurement will be described with reference to FIG.10(a). FIG. 10(a) is a schematic cross-sectional view illustrating adiffraction of X-rays with respect to the curved c-plane.

The X-ray irradiation width b on the main surface of the substrate iscalculated by the following formula (h).

b=a/sin θ_(B)  (h)

wherein a slit width at the incident side of the X-ray is a, an X-rayirradiation width (footprint) irradiated on the main surface of thesubstrate is b, and a crystal Bragg angle is θ_(B).

As illustrated in in FIG. 10(a), when the c-plane of the substrate iscurved, the radius of curvature R of the c-plane is very large withrespect to the X-ray irradiation width b, wherein the radius ofcurvature of the c-plane is R and half of the central angle formed bythe curved c-plane is γ in a range of the X-ray irradiation width b.Therefore, the angle γ can be obtained by the following equation (i).

γ=sin⁻¹(b/2R)≈b/2R  (i)

At this time, at the incident side end (right end in the figure) of theregion on the c-plane of the substrate irradiated with X-rays, thediffraction angle with respect to the main surface of the substrate isθ_(B)+γ=θ_(B)+b/2R.

On the other hand, at the receiving side end (left end in the figure) ofthe region on the c-plane of the substrate irradiated with X-rays, thediffraction angle with respect to the main surface of the substrate isθ_(B)−γ=θ_(B)−b/2R.

Accordingly, based on the difference between the diffraction angle withrespect to the main surface of the substrate at the incident side end onthe c-plane of the substrate and the diffraction angle with respect tothe main surface of the substrate at the light receiving side end on thec-plane of the substrate, the fluctuation of the X-ray diffraction anglewith respect to the curved c-plane is b/R.

FIGS. 10(b) and 10(c) are views illustrating the fluctuation of thediffraction angle of the (0002) plane with respect to the radius ofcurvature of the c plane. The vertical axis of FIG. 10(b) is alogarithmic scale, and the vertical axis of FIG. 10(c) is a linearscale.

As illustrated in FIGS. 10(b) and 10(c), when the width a of the slit atthe incident side of the X-ray is increased, that is, when theirradiation width b of the X-ray is increased, the fluctuation of thediffraction angle of the (0002) plane increases according to the X-rayirradiation width b. As the radius of curvature R of the c-plane becomessmaller, the fluctuation of the diffraction angle of the (0002) planegradually increases. The difference in the fluctuation of thediffraction angle of the (0002) plane in the case of different X-rayirradiation width b, becomes larger as the radius of curvature R of thec plane becomes smaller.

When the slit width at the incident side is narrow, the influence of thecurvature of the c-plane is small, and the influence of theabove-described crystal quality factor becomes dominant in thefluctuation of the diffraction angle of the (0002) plan. However, whenthe width a of the slit at the incident side is wide, both the influenceof the above-described crystal quality factor and the influence of thecurvature of the c-plane will be superimposed, in the fluctuation of thediffraction angle of the (0002) plane. Accordingly, when the X-raylocking curve measurement is performed with a different slit width a atthe incident side, both the above-described crystal quality factor andthe curvature (warp) of the c-plane can be evaluated at the same timeover the region irradiated with X-rays.

Here, the features of the substrate 50 of the present embodiment at thetime of performing the X-ray locking curve measurement, will bedescribed.

In the following, in the case where the main surface 50 s of thesubstrate 50 is irradiated with (Cu) Kα1 X-rays through a two-crystalmonochromator of Ge (220) plane and a slit to measure the X-ray lockingcurve of the (0002) plane diffraction, the full width at half maximum ofthe (0002) plane diffraction is “FWHMa” when a slit width in ω directionis 1 mm, and the full width at half maximum of the (0002) planediffraction is “FWHMb” when a slit width in ω direction is 0.1 mm. The“ω direction” refers to a rotation direction when the substrate 50 isrotated about an axis parallel to the main surface of the substrate 50and passing through the center of the substrate 50 in the X-ray lockingcurve measurement.

In the substrate 50 of the present embodiment, all of the crystalquality factors such as high/low dislocation density, high/lowmosaicity, high/low stacking fault density, high/low basal planedislocation density, high/low point defect density (vacancy, etc.),large or small amount of in-plane fluctuation of lattice constant, and adistribution of an impurity concentration, are good.

As a result, in the substrate 50 of the present embodiment, the X-raylocking curve measurement for the (0002) plane diffraction when the slitwidth in ω direction is 0.1 mm, reveals that full width at half maximumFWHMb of the (0002) plane diffraction is, for example, 80 arcsec orless, preferably 50 arcsec or less, and more preferably 32 arcsec orless.

Further, as described above, the substrate 50 of the present embodiment,all of the above-described crystal quality factors are good over a widerange of the main surface 50 s.

As a result, the X-ray locking curve measurement for the (0002) planediffraction when the slit width in ω direction is 0.1 mm, at a pluralityof measurement points set at intervals of 5 mm (between the center andthe outer edge) within the main surface 50 s of the substrate 50 of thepresent embodiment, reveals that full width at half maximum FWHMb of the(0002) plane diffraction is 80 arcsec or less, preferably 50 arcsec orless, and more preferably 32 arcsec or less, for example, at 90% or moreof all measurement points.

Further, in the substrate 50 of the present embodiment, the in-planevariation of the above-described crystal quality factors is small.Therefore, it is found that the diffraction pattern of the (0002) planewhen the X-ray locking curve measurement is performed with the slitwidth at the incident side widened, is less likely to be narrow than thediffraction pattern of the (0002) plane when the X-ray locking curvemeasurement is performed with the slit width at the incident sidenarrowed.

As a result, in the substrate 50 of the present embodiment, full widthat half maximum FWHMb of the (0002) plane diffraction when a slit widthin ω direction is 1 mm, can be, for example, full width at half maximumFWHMb or more of the (0002) plane diffraction when a slit width in wdirection is 0.1 mm.

Even when the crystal quality factor of the substrate 50 is good, thereis a case of FWHMa<FWHMb, with FWHMb being very small.

Further, in the substrate 50 of the present embodiment, as describedabove, not only are there few dislocations, but all of theabove-described crystal quality factors are well-balanced and good, overa wide range of the main surface 50 s. Further, the curvature of thec-plane 50 c of the substrate 50 is small, and the radius of curvatureof the c-plane 50 c is large. Therefore, even when the X-ray lockingcurve is measured by widening the slit width at the incident side in thesubstrate 50 of the present embodiment, the fluctuation of thediffraction angle of the (0002) plane does not become large, because theabove-described crystal quality factors are well-balanced and good, andthe radius of curvature of the c-plane is large, over the regionirradiated with X-rays. Therefore, even when the X-ray locking curvemeasurement is performed with a different slit width at the incidentside, the difference in the fluctuation of the diffraction angle of the(0002) plane becomes small.

As a result, difference FWHMa−FWHMb obtained by subtracting FWHMb fromFWHMa is for example, 30% or less, preferably 22% or less of FWHMa, at apredetermined measurement point (for example, the center of the mainsurface) of the substrate 50 of the present embodiment, wherein FWHMa isfull width at half maximum of the (0002) plane diffraction when a slitwidth in ω direction is 1 mm, and FWHMb is full width at half maximum ofthe (0002) plane diffraction when a slit width in ω direction is 0.1 mm.

In the substrate 50 of the present embodiment, FWHMa−FWHMb/FWHMa is 30%or less, even in the case of FWHMa<FWHMb. Further, in the substrate 50of the present embodiment, FWHMa may be substantially equal to FWHMb,and |FWHMa−FWHMb|/FWHMa may be 0%.

Further, in the substrate 50 of the present embodiment, even when theX-ray locking curve measurement is performed with a slit width at theincident side widened, the diffraction pattern has a single peak due tothe small variation of the above-described crystal quality factors overthe region irradiated with X-rays.

For a reference, a substrate manufactured by a conventionalmanufacturing method (hereinafter, also referred to as a conventionalsubstrate) will be described. The conventional manufacturing methodsreferred to herein are, for example, a conventional VAS method, a methodof growing a thick film using the c-plane as a growth surface, theabove-described DEEP method, THVPE (Tri-halide phase epitaxy) method,ammonothermal method, flux method, and the like.

In the conventional substrate, at least one of the crystal qualityfactors described above is not better than that of the substrate 50 ofthe present embodiment. Therefore, FWHMb in the conventional substrateis larger than that of the substrate 50 of the present embodiment.

In the conventional substrate, in-plane variability in at least one ofthe crystal quality factors described above can occur. Therefore, thediffraction pattern of the (0002) plane when the X-ray locking curvemeasurement is performed with the slit width at the incident sidewidened, may be wider than the diffraction pattern of the (0002) planewhen the X-ray locking curve measurement is performed with the slitwidth at the incident side narrowed. As a result, in the conventionalsubstrate, FWHMa<FWHMb may be satisfied.

In the conventional substrate, the radius of curvature of the c-plane issmaller than that of the substrate 50 of the present embodiment. Whenthe slit width is widened, at least a part of the region irradiated withX-rays necessarily includes a portion where at least one of the crystalquality factors is not better than that of the substrate 50 of thepresent embodiment. Therefore, difference FWHMa−FWHMb in the basesubstrate 10 is larger than that of the substrate 50 of the presentembodiment.

In the conventional substrate, in-plane variability in at least one ofthe crystal quality factors described above can occur. When the slitwidth is widened, there may be places where the fluctuation of thediffraction angle is different in at least a part of the regionirradiated with X-rays. Therefore, the diffraction pattern may have aplurality of peaks when the slit width is widened.

As described above, the conventional substrate may not satisfy theabove-described conditions defined for the substrate 50 of the presentembodiment.

(4) Effect Obtained by the Present Embodiment

According to the present embodiment, one or more of the followingeffects can be obtained.

(a) In the first step S200, since the inclined interface 30 i other thanthe c-plane is generated on the surface of a single crystal constitutingthe first layer 30, the dislocations can be bent and propagated in adirection substantially perpendicular to the inclined interface 30 i, ata position where the inclined interface 30 i is exposed. Thereby, thedislocations can be collected locally. Since the dislocations arecollected locally, the dislocations having Burgers vectors that areopposite to each other can disappear. Alternatively, since the locallycollected dislocations form a loop, the dislocations can be preventedfrom propagating to the surface side of the second layer 40. In thisway, the dislocation density in the surface of the second layer 40 canbe lowered. As a result, it is possible to obtain the substrate 50having a dislocation density lower than that of the base substrate 10.(b) As described above, since some of the plurality of dislocationsdisappears and some of the plurality of dislocations are suppressed frompropagating to the surface side of the second layer 40, etc., during thegrowth process of the second layer 40, the dislocation density can belowered sharply and faster than in the case of the c-plane limitedgrowth. That is, the reduction rate of the dislocation density obtainedby N/N₀ in the present embodiment can be made smaller than the reductionrate of the dislocation density obtained by N′/N₀ in the case of thec-plane limited growth. As a result, the substrate 50 having a lowerdislocation density than that of the base substrate 10 can beefficiently obtained, and its productivity can be improved.(c) In the first step S200, the c-plane 30 c disappears from the topsurface 30 u of the first layer 30. Thereby, a plurality of valleys 30 vand a plurality of tops 30 t can be formed on the surface of the firstlayer 30. As a result, the dislocations propagating from the basesubstrate 10 can be reliably bent at the position where the inclinedinterface 30 i in the first layer 30 is exposed.

Here, a case where the c-plane remains in the first step will beconsidered. In this case, in the portion where the c-plane remains, thedislocations propagated from the base substrate propagate substantiallyvertically upward without being bent and reach the surface of the secondlayer. Therefore, in the upper part of the portion where the c-planeremains, dislocations are not lowered and a high dislocation densityregion is formed.

In contrast, according to the present embodiment, since the c-plane 30 cdisappears from the top surface 30 u of the first layer 30 in the firststep S200, the surface of the first layer 30 can be formed only by theinclined interface 30 i other than the c-plane, and a plurality ofvalleys 30 v and a plurality of tops 30 t can be formed on the surfaceof the first layer 30. Thereby, the dislocations propagating from thebase substrate 10 can be reliably bent over the entire surface of thefirst layer 30. Since the dislocations are reliably bent, some of theplurality of dislocations can be easily disappears, or some of theplurality of dislocations is hardly propagated to the surface side ofthe second layer 40. As a result, the dislocation density can be loweredover the entire main surface 1 s of the substrate 50 obtained from thesecond layer 40.

(d) In the present embodiment, by setting the RMS of the main surface 10s of the base substrate 10 to 1 nm or more, it is possible to promotethe generation of the inclined interface 30 i other than the c-plane onthe surface of the first layer 30, when the first layer 30 is grown onthe base substrate 10 in the first step S200.

Further, in the present embodiment, the crystal strain introduced byprocessing of the base substrate 10 is left on the main surface 10 sside of the base substrate 10. At this time, full width at half maximum(FWHM) of the (10-10) plane surface diffraction when X-ray locking curvemeasurement is performed with the incident angle with respect to themain surface 10 s of the base substrate 10 after processing set to 2°,is made larger than the full width at half maximum of the base substrate10 before processing, and is set to 60 arcsec or more. Thereby, a stablecrystal plane appearing on the surface of the first layer 30 due to thecrystal strain on the main surface 10 s side of the base substrate 10,can be changed. As a result, the inclined interface 30 i other than thec-plane can be generated on the surface of the first layer 30.

(e) In the present embodiment, since the first growth condition isadjusted so as to satisfy the formula (1) using the above-described basesubstrate 10 in the first step S200, {11-2m} plane satisfying m 3 can begenerated as the inclined interface 30 i in the first step S200.Thereby, the inclination angle of the {11-2m} plane with respect to thec plane 30 c can be loose. Specifically, the inclination angle can be47.3° or less. Since the inclination angle of the {11-2m} plane withrespect to the c plane 30 c is loose, a cycle of the plurality of tops30 t can be lengthened. Specifically, the average distance L between theclosest tops can be more than 100 μm, when observing an arbitrary crosssection perpendicular to the main surface 10 s of the base substrate 10.

For a reference, usually, when an etch pit is generated in a nitridesemiconductor substrate using a predetermined etchant, an etch pitincluding the {1-10n} plane is formed on the surface of the substrate.In contrast, on the surface of the first layer 30 grown under apredetermined condition in the present embodiment, the {11-2 m} planesatisfying m 3 can be generated. Accordingly, it is considered that theinclined interface 30 i peculiar to the manufacturing method is formedin the present embodiment as compared with a normal etch pit.

(f) In the present embodiment, when observing an arbitrary cross sectionperpendicular to the main surface 10 s of the base substrate 10, sincethe average distance L between the closest tops is more than 100 μm, atleast over 50 μm distance can be secured for bending and propagating thedislocations. Thereby, the dislocations can be sufficiently collected inthe upper part of the substantially center between the pair of tops 30 tof the first layer 30. As a result, the dislocation density in thesurface of the second layer 40 can be sufficiently lowered.(g) In the first step S200, after the c-plane 30 c disappears from thesurface of the first layer 30, the growth of the first layer 30 iscontinued over a predetermined thickness, while maintaining the statewhere the surface is composed only of the inclined interface 30 i.Thereby, the c-plane 30 c can reliably disappears over the entiresurface of the first layer 30. For example, even if the timing is off atwhich the c-plane 30 c disappears on the surface of the first layer 30in the inclined interface expansion step S220 and the c-plane 30 cremains on a part of the expanded inclined interface layer 32, thec-plane 30 c can reliably disappear.

Further, due to continuing of the growth of the first layer 30 at theinclined interface 30 i after the c-plane 30 c disappears, a sufficienttime can be secured to bend the dislocations at the position where theinclined interface 30 i is exposed. Here, when the c-plane growsimmediately after the c-plane disappears, there is a possibility thatthe dislocations are not sufficiently bent and propagate in thesubstantially vertical direction toward the surface of the second layer.In contrast, according to the present embodiment, since sufficient timeis secured to bend the dislocations at the position where the inclinedinterface 30 i other than the c-plane is exposed, particularly, thedislocations near the top 30 t of the first layer 30 can be reliablybent, and the propagation of dislocations in the substantially verticaldirection from the base substrate 10 toward the surface of the secondlayer 40, can be suppressed. Thereby, the concentration of thedislocations in the upper part of the top 30 t of the first layer 30 canbe suppressed.

(h) According to the manufacturing method of the present embodiment, theradius of curvature of the c-plane 50 c of the substrate 50 can belarger than the radius of curvature of the c-plane 10 c of the basesubstrate 10. Thereby, the variation in the off-angle θ of the c-axis 50ca with respect to the normal of the main surface 50 s of the substrate50 can be smaller than the variation in the off-angle of the c-axis 10ca of the substrate 10.

As one of the reasons why the radius of curvature of the c-plane 50 c ofthe substrate 50 can be large, for example, the following reasons can beconsidered.

As described above, in the first step S200, the inclined interfacegrowth region 70 is formed by three-dimensionally growing the firstlayer 30 with the inclined interface 30 i other than the c-plane as agrowth surface. In the inclined interface growth region 70, oxygen iseasily taken in as compared with the first c-plane growth region 60.Therefore, the oxygen concentration in the inclined interface growthregion 70 is higher than the oxygen concentration in the first c-planegrowth region 60. That is, the inclined interface growth region 70 canbe considered as a high oxygen concentration region.

As described above, by taking the oxygen into the high oxygenconcentration region, the lattice constant of the high oxygenconcentration region can be larger than the lattice constant of otherregions other than the high oxygen concentration region. (Reference:Chris G. Van de Walle, Physical Review B vol. 68, 165209 (2003)). Due tothe curvature of the c-plane 10 c of the base substrate 10, stressconcentrated toward the center of the curvature of the c-plane isapplied on the base substrate 10 or the first c-plane growth region 60grown with the c-plane 30 c or the first layer 30 as a growth surface.In contrast, by relatively increasing the lattice constant in the highoxygen concentration region, a stress can be generated in the highoxygen concentration region so as to spread the c-plane 30 c outward ina creepage direction. Thereby, the stress concentrated toward the centerof curvature of the c-plane 30 c on the lower side of the high oxygenconcentration region, and the stress that spreads the c-plane 30 c inthe high oxygen concentration region outward in the creepage direction,can be offset.

As described above, due to the stress offset effect of the first layer30, the radius of curvature of the c-plane 50 c of the substrate 50obtained from the second layer 40 can be larger than the radius ofcurvature of the c-plane 10 c of the substrate 10.

(i) In the substrate 50 obtained by the manufacturing method of thepresent embodiment, the dislocation density can be lowered, and not onlycan the off-angle variation be reduced, but all of the above-describedcrystal quality factors that determine the full width at half maximum ofthe X-ray locking curve measurement can be well-balanced and good.Thereby, in the substrate 50 of the present embodiment, FWHMb can be 32arcsec or less. Further, in the substrate 50 of the present embodiment,the radius of curvature of the c-plane is large, and the above-describedcrystal quality factors are well-balanced and good over the entireregion irradiated with X-rays, even when the slit width is 1 mm.Therefore, (FWHMa−FWHMb)/FWHMa can be 30% or less.

Other Embodiments

As described above, the embodiments of the present disclosure have beenspecifically described. However, the present disclosure is not limitedto the above-described embodiments, and various modifications can bemade without departing from the gist thereof.

In the above-described embodiment, explanation is given for the casewhere the base substrate 10 is a GaN free-standing substrate. However,the base substrate 10 is not limited to the GaN free-standing substrate,and for example, may be a free-standing substrate comprising a group IIInitride semiconductor such as aluminum Nitride (AlN), indium galliumnitride (AlGaN), indium nitride (InN), indium gallium nitride (InGaN),indium gallium nitride (AlInGaN), that is, a free-standing substratecomprising a group III nitride semiconductor represented by acomposition formula of Al_(x)In_(y)Ga_(1-x-y)N (0≤x≤1, 0≤y≤1, 0≤x+y≤1).

In the above-described embodiment, explanation is given for the casewhere the substrate 50 is a GaN free-standing substrate. However, thesubstrate 50 is not limited to the GaN free-standing substrate, and forexample, may be a free-standing substrate comprising a group III nitridesemiconductor such as AlN, AlGaN, InN, InGaN, AlInGaN, that is, a groupIII nitride semiconductor represented by a composition formula ofAl_(x)In_(y)Ga_(1-x-y)N (0≤x≤1, 0≤y≤1, 0≤x+y≤1).

In the above-described embodiment, explanation is given for the casewhere the substrate 50 is n-type. However, the substrate 50 may bep-type or may have semi-insulating properties. For example, whenmanufacturing a semiconductor device as a high electron mobilitytransistor (HEMT) using the substrate 50, the substrate 50 preferablyhas semi-insulating properties.

In the above-described embodiment, explanation is given for the casewhere the growth temperature is mainly adjusted as the first growthcondition in the first step S200. However, when the first growthcondition satisfies the formula (1), the growth condition other than thegrowth temperature may be adjusted, or the growth temperature and thegrowth condition other than the growth temperature may be adjusted incombination, as a first growth condition.

In the above-described embodiment, explanation is given for the casewhere the growth temperature is mainly adjusted as the second growthcondition in the second step S300. However, when the second growthcondition satisfies the formula (2), the growth condition other than thegrowth temperature may be adjusted, or the growth temperature and thegrowth condition other than the growth temperature may be adjusted incombination, as a second growth condition.

In the above-described embodiment, explanation is given for the casewhere the growth condition in the inclined interface maintenance stepS240 is maintained under the above-described first growth condition asin the inclined interface expansion step S220. However, when the growthcondition in the inclined interface maintenance step S240 satisfy thefirst growth condition, the growth condition in the inclined interfacemaintenance step S240 may be different from the growth condition in theinclined interface expansion step S220.

In the above-described embodiment, explanation is given for the casewhere the growth condition in the main growth step S340 is maintainedunder the above-described second growth condition as in the c-planeexpansion step S320. However, when the growth condition in the maingrowth step S340 satisfies the second growth condition, the growthcondition in the main growth step S340 may be different from the growthcondition in the c-plane expansion step S320.

In the above-described embodiment, explanation is given for the casewhere the second crystal layer 6 or the main growth layer 44 is slicedusing a wire saw in the slicing step S170 and the slicing step S400.However, for example, an outer peripheral blade slicer, an innerperipheral blade slicer, an electric discharge machine, or the like maybe used.

In the above-described embodiment, explanation is given for the casewhere the substrate 50 is obtained by slicing the main growth layer 44in the laminated structure 90. However, the present disclosure is notlimited thereto. For example, the laminated structure 90 may be used asit is to manufacture a semiconductor laminate for manufacturing asemiconductor device. Specifically, after preparing the laminatedstructure 90, in the semiconductor laminate manufacturing step, asemiconductor functional layer is epitaxially grown on the laminatedstructure 90 to prepare a semiconductor laminate. After preparing thesemiconductor laminate, a back surface side of the laminated structure90 is polished, and the base substrate 10, the first layer 30, and thec-plane expanded layer 42 are removed from the laminated structure 90.Thereby, a semiconductor laminate having the main growth layer 44 andthe semiconductor functional layer can be obtained as in theabove-described embodiment. According to this case, the slicing stepS400 and the polishing step S500 for obtaining the substrate 50 can beomitted.

In the above-described embodiment, explanation is given for the casewhere the manufacturing step is completed after the substrate 50 ismanufactured. However, steps S200 to S500 may be repeated, using thesubstrate 50 as the base substrate 10. Thereby, the substrate 50 havinga further lowered dislocation density can be obtained. Further, thesubstrate 50 with further reduced variation in the off-angle θ of thec-axis 50 ca, can be obtained. Further, the steps S200 to S500 using thesubstrate 50 as the base substrate 10 may be set as one cycle, and thecycle may be repeated a plurality of times. Thereby, the dislocationdensity of the substrate 50 can be gradually lowered according to thenumber of times the cycle is repeated. Further, the variation in theoff-angle θ of the c-axis 50 ca of the substrate 50 can be graduallyreduced according to the number of times the cycle is repeated.

EXAMPLE

Hereinafter, various experimental results supporting the effects of thepresent disclosure will be described. In the following, the “nitridesemiconductor substrate” may be simply abbreviated as the “substrate”.

(1) Experiment 1 (1-1) Preparation of the Nitride SemiconductorSubstrate

The substrates of an example and a comparative example were prepared asfollows. For an example, a laminated structure before slicing thesubstrate was also prepared.

[Conditions for Preparing the Nitride Semiconductor Substrate of anExample] (Base Substrate) Material: GaN

Preparation method: VAS methodDiameter: 2 inches

Thickness: 400 μm

Crystal plane with the lowest index closest to the main plane: c planeNo pattern processing such as formation of the mask layer on the mainsurface.Root mean square roughness RMS of the main surface: 2 nmOff-angle of the main surface: 0.4° in m-directionFWHM of (10-10) plane diffraction in XRC measurement: 100 arcsec(First layer)

Material: GaN

Growth method: HVPE methodFirst growth condition:

The growth temperature was 980° C. or higher and 1,020° C. or lower, andthe V/III ratio was 2 or higher and 20 or lower. At this time, at leastone of the growth temperature and the V/III ratio was adjusted withinthe above range so that the first growth condition satisfied the formula(1).

(Second layer)

Material: GaN

Growth method: HVPE methodGrowth temperature: 1,050° C.V/III ratio: 2The second growth condition satisfies the formula (2).Thickness from the main surface of the base substrate to the surface ofthe second layer: approximately 2 mm(Slice condition)Substrate thickness: 400 μm

Carfloss: 200 μm

In the example, two substrates having slightly different processingstates were prepared.

[Conditions for Preparing Nitride Semiconductor Substrate in ComparativeExample] (Base Substrate) Material: GaN

Preparation method: VAS methodDiameter: 2 inches

Thickness: 400 μm

Crystal plane with a lowest index closest to the main surface: c-planeNo pattern processing such as mask layer on the main surface.Root mean square roughness RMS of the main surface: 0.7 nmOff-angle of the main surface: 0.4° in m-directionFWHM of (10-10) plane diffraction in XRC measurement: 50 arcsec(Crystal layer)

Material: GaN

Growth method: HVPE methodGrowth temperature: 1,050° C. (same as the second layer of the example)V/III ratio: 2 (same as the second layer of the example)The above growth conditions satisfy formula (2).Thickness from the main surface of the base substrate to the surface ofthe crystal layer: 2 mm(Slice condition)Same as the example.

(1-2) Evaluation (Observation Using a Fluorescence Microscope)

Using a fluorescence microscope, the cross section of the laminatedstructure before slicing the substrate was observed in the example.

(Observation Using a Multiphoton Excitation Microscope)

The main surfaces of the base substrate, the substrate of the example,and the substrate of the comparative example were observed respectively,using a multiphoton excitation microscope. At this time, the dislocationdensity was measured by measuring a dark spot density over the entiremain surface every 250 μm of the field of view. It is confirmed bymeasuring by shifting a focus in a thickness direction that all darkspots on these substrates are dislocations. Further, at this time, theratio of the number of regions having a dislocation density of less than1×10⁶ cm⁻² (low dislocation density region) with respect to the totalnumber of measurement regions in a 250 μm square field of view wasobtained. The “low dislocation density region” referred to herein, asshown in the results below, means a region having a dislocation densitylower than an average dislocation density in the main surface of thecrystal layer of the comparative example in which the crystal layer wasgrown without performing the first step.

(X-Ray Locking Curve Measurement)

The following two types of X-ray locking curve measurements wereperformed for each of the base substrate, the substrate of the example,and the substrate of the comparative example.

For the X-ray locking curve measurement, “X'Pert-PRO MRD” manufacturedby Spectris was used, and “Hybrid monochromator” manufactured by thesame company was used as the monochromator at the incident side. Thehybrid monochromator has an X-ray mirror and two crystals of Ge (220)plane in this order from an X-ray light source side. In the measurement,first, the X-rays emitted from the X-ray light source are made intoparallel light by an X-ray mirror. Thereby, the number of used X-rayphotons (ie, X-ray intensity) can be increased. Next, a parallel lightfrom the X-ray mirror is made into (Cu) Kα1 monochromatic light by twocrystals of Ge (220) plane. Next, the monochromatic light from the twocrystals of Ge (220) plane is narrowed to a predetermined width throughthe slit and incident on the substrate. When the full width at halfmaximum is obtained by simulation at the time of measuring the lockingcurve of the (0002) plane of a perfect crystal GaN using the hybridmonochromator, it is 25.7 arcsec. That is, the full width at halfmaximum is a theoretical measurement limit when measuring with theabove-described optical system.

In the measurement, the X-rays incident on the substrate are parallellights toward the substrate side in the cross section along ω direction,but are not parallel lights in the cross section along a directionorthogonal to w direction (a direction of a rotation axis of thesubstrate). Therefore, the width of the X-ray in ω direction is almostconstant, but the width of the X-ray in the direction orthogonal to ωdirection increases while the X-rays reaching the substrate from theslit. Accordingly, in the X-ray locking curve measurement, the fullwidth at half maximum of the X-rays diffracted at a predeterminedcrystal plane, depends on the slit width at the incident side in ωdirection in which the X-rays are parallel light.

On the other hand, the light receiving side was open. A window width ofa detector on the light receiving side was 14.025 mm. In theabove-described optical system, since a goniometer radius is 420 mm, itis possible to measure the fluctuation of the Bragg angle of ±0.95°.

(X-Ray Locking Curve Measurement 1)

The X-ray locking curve of the (0002) plane of each of the basesubstrate, the substrate of the example, and the substrate of thecomparative example, was measured, with a slit width at the incidentside in ω direction set to 0.1 mm. At this time, the measurement wasperformed at a plurality of measurement points set at intervals of 5 mmin each of the m-axis direction and the a-axis direction in the mainsurface of each substrate. As a result of the measurement, the radius ofcurvature of the c-plane and the off-angle, which is the angle formed bythe c-axis with respect to the normal of the main surface, were obtainedbased on the diffraction peak angle on the (0002) plane at eachmeasurement point. Further, the variation in the off-angle was obtainedas a difference between maximum and minimum in the size of the off-anglewithin a diameter of 29.6 mm from the center of the main surface.Further, FWHMb was obtained at each measurement point, FWHMb being fullwidth at half maximum of the (0002) plane diffraction when the slitwidth at the incident side in ω direction was 0.1 mm.

(X-Ray Locking Curve Measurement 2)

The X-ray locking curve was measured for each of the base substrate andthe substrate of the example, with a slit width at the incident side inω direction set to 1 mm. The measurement was performed at the center ofthe main surface of each substrate. As a result of the measurement, fullwidth at half maximum FWHMa of the (0002) plane diffraction wasobtained, with the slit width at the incident side in ω direction set to1 mm. Further, the ratio of FWHMa−FWHMb to FWHMa was obtained at thecenter of the main surface of each substrate.

In the X-ray locking curve measurements 1 and 2, when the X-rays areincident on the main surface of each substrate at a Bragg angle of17.28° of the (0002) plane with respect to the main surface, X-rayfootprint with a slit width in ω direction set to 0.1 mm is about 0.337mm, and X-ray foot print with a slit width in ω direction set to 1 mm isabout 3.37 mm.

(1-3) Result

The results are shown in Table 1.

TABLE 1 Example Com. Ex. A B 4.5 × 10⁵ 1.5 × 10⁶ 3.0 × 10⁶ C 95 30 0   D33.0~68.2 11.3 7.64 E 0.025~0.052 0.15 0.22 F 26.2~31.5 38.5~66.240.1~77.8 G  4.5~28.1 — 54.2~79.6 Com. Ex. = Comparative Example A =Base substrate B = Average dislocation density (cm⁻²) C = Percentage oflow dislocation density region (%) D = Radius of curvature of c-plane(m) E = Variation in off-angle within a diameter of 29.6 mm (°) F =(In-plane) FWHMb (arcsec) G = (FWHMa − FWHMb)/FWHMa (%)

FIG. 11 is a view illustrating an observation image obtained byobserving a cross section of the laminated structure of the exampleusing a fluorescence microscope. As illustrated in FIG. 11, in thelaminated structure of the example, the first layer had the firstc-plane growth region grown with the c-plane as a growth surface and theinclined interface growth region grown with the inclined interface as agrowth surface, based on a difference in growth surfaces during thegrowth process (ie, difference in oxygen concentration). The firstc-plane growth region had a plurality of concave portions and aplurality of convex portions. An average value of the angle formed bythe pair of inclined portions in the first c-plane growth region wasapproximately 52°. Further, the average distance between the closesttops was approximately 234 μm. Further, the height from the main surfaceof the base substrate to the top of the first c-plane growth region wasapproximately 298 to 866 μm. Further, the inclined interface growthregion was continuously formed along the main surface of the basesubstrate. Further, the thickness of the boundary surface of the secondlayer at the position where the inclined interface disappeared was about1 mm from the main surface of the base substrate.

As illustrated in table 1, in the substrate of the example, the averagedislocation density in the main surface was significantly lowered ascompared with the base substrate and the substrate of the comparativeexample, and was less than 5.5×cm⁻². Even when the crystal layer wasgrown thick as in the comparative example, the dislocation density ofthe substrate was lower than that of the base substrate, but in thesubstrate of the example, the dislocation density was further lowered ascompared with the comparative example.

Further, a lowering rate of the dislocation density obtained by N/N₀described above was 0.15, wherein the dislocation density of thesubstrate of the example was N.

Further, in the substrate of the example, there was no region where thedislocation density exceeded 3×10⁶ cm⁻². Even in the region with ahighest dislocation density, the dislocation density was less than1.5×10⁶ cm⁻². Further, in the substrate of the example, a region havinga dislocation density of less than 1×10⁶ cm⁻² (low dislocation densityregion) exists at 90% or more of the main surface 50 s. The dislocationdensity in the low dislocation density region was 1.7×10⁵ to 8.1×10⁵cm⁻².

Further, as shown in table 1, in the substrate of the example. theradius of curvature of the c-plane was larger than that of the basesubstrate and the substrate of the comparative example, and was 22 m ormore. Further, in the substrate of the example, the variation in theoff-angle of the c-axis within the diameter of 29.6 mm was reduced ascompared with the base substrate and the substrate of the comparativeexample, and was 0.075° or less. Even when the crystal layer was grownthick as in the comparative example, the variation in the off-angle ofthe c-axis in the substrate was smaller than that in the base substrate,but in the substrate of the example, the variation in the off-angle ofthe c-axis was further smaller than that of the comparative example.

Further, as shown in table 1, in the substrate of the example, FWHMb ofthe (0002) plane diffraction was 32 arcsec or less at all measurementpoints (that is, 100%), FWHMb being full width at half maximum when thewidth of the slit in ω direction was 0.1 mm.

FIG. 12(a) is a view illustrating a normalized X-ray diffraction patternof the substrate of the example when an X-ray locking curve is measuredwith a different slit, and (b) is a view illustrating the normalizedX-ray diffraction pattern when the same measurement as in the example isperformed for the base substrate. FIGS. 12(a) and 12(b) illustrate themeasurement results in a direction along the m-axis. Further, in thesefigures, “Line with” means the above-described X-ray footprint.

As illustrated in FIG. 12(b), in the base substrate, when the slit widthin ω direction was 0.1 mm, the X-ray diffraction pattern was narrow, butwhen the slit width in ω direction was 1 mm, the X-ray diffractionpattern was widened.

Therefore, as shown in table 1, in the base substrate, FWHMa-FWHMb was50% or more of FWHMa.

In contrast, as illustrated in FIG. 12(a), in the substrate of theexample, even when the slit width in ω direction was widened from 0.1 mmto 1 mm, the X-ray diffraction pattern was slightly widened, but thespread was small.

Thereby, as shown in table 1, in the substrate of the example,FWHMa−FWHMb was 0% or more and 30% or less of FWHMa.

According to the above examples, the root mean square roughness RMS ofthe main surface of the base substrate was 1 nm or more, and theoff-angle of the main surface of the base substrate was 0.4° or less.Further, the crystal strain introduced by processing of the basesubstrate was left on the main surface side of the base substrate, andFWHM of the (10-10) plane diffraction in the XRC measurement of the basesubstrate after processing was 60 arcsec or more. Thereby, it waspossible to sufficiently promote the generation of an inclined interfaceother than the c-plane on the surface of the first layer. Further, inthe first step, the first growth condition was adjusted so as to satisfythe formula (1). Thereby, in the growth process of the first layer, thec-plane could be reliably disappeared. By reliably making the c-planedisappear, the dislocations could be reliably bent at the position wherethe inclined interface in the first layer was exposed. As a result, itwas confirmed that the dislocation density in the main surface of thesubstrate could be lowered efficiently.

Further, according to the example, it was confirmed that the radius ofcurvature of the c-plane of the substrate could be made larger than theradius of curvature of the c-plane of the substrate, and the variationin the off-angle of the c-axis on the substrate and the variation in theoff-angle of the c-axis on the base substrate, could be reduced.

Further, according to the example, as described above, there were fewdislocations over a wide range of the main surface of the substrate, andall of the crystal quality factors in the substrate were well-balancedand good. Thereby, in the substrate of the example, it was confirmedthat FWHMb was 32 arcsec or less over a wide range of the main surface.

Further, according to the example, as described above, all of thecrystal quality factors were well-balanced and good, and the radius ofcurvature of the c-plane of the substrate was large. Thereby, in theexample, it was confirmed that the difference FWHMa−FWHMb was 30% orless of FWHMa, FWHMa and FWHMb being full width at half maximum when theX-ray locking curve was measured with a different slit width at theincident side.

(2) Experiment (2-1) Preparation of a Laminated Structure

In order to investigate the inclined interface generated on the surfaceof the first layer, a laminated structure having a base substrate, afirst layer and not having a second layer was prepared. The conditionsfor the base substrate and the first layer were almost the same as thoseof the example of experiment 1.

(2-2) Evaluation (Observation Using an Optical Microscope)

The surface of the first layer of the laminated structure was observedusing an optical microscope.

(Observation Using a Fluorescence Microscope)

The cross section of the laminated structure was observed using afluorescence microscope.

(2-3) Result

FIG. 13(a) is a view illustrating an observation image obtained byobserving the surface of the laminated structure of experiment 2 usingan optical microscope, and (b) is a view illustrating an observationimage obtained by observing the surface of the laminated structure ofexperiment 2 using a scanning electron microscope. FIG. 14(a) is a viewillustrating an observation image obtained by observing the M-crosssection of the laminated structure of experiment 2 using an opticalmicroscope, and (b) is a view illustrating an observation image obtainedby observing the M-cross section of the laminated structure ofexperiment 2 using a scanning electron microscope. FIG. 15(a) is a viewillustrating an observation image obtained by observing the a-crosssection of the laminated structure of experiment 2 using an opticalmicroscope, and (b) is a view illustrating an observation image obtainedby observing the a-cross section of the laminated structure ofexperiment 2 using a scanning electron microscope.

As illustrated in FIGS. 13(a) to 15(b), a plurality of concaves formedby the inclined interfaces other than the c-plane were generated on thetop surface of the first layer.

As illustrated in FIG. 13(a), six shining visible surfaces were formedin the concave generated on the top surface of the first layer, that is,the concave had six inclined interfaces.

As illustrated in FIG. 13(b), six ridge lines (an example indicated bywhite lines) in the concaves generated on the top surface of the firstlayer, were formed evenly from the center. That is, the concave was inthe form of an inverted regular hexagonal cone. Further, whenconsidering the direction of the orientation flat of the base substrate,there is the ridgeline in the concave along the <1-100> axial direction,and the inclined interface constituting the concave was a plane (thatis, {11-2 m} plane) whose normal direction was a direction inclined fromthe <11-20> axis.

The M-cross section (cross section in the direction along the <11-20>axis) illustrated in FIGS. 14(a) and (b) is obtained by almostvertically cutting the inclined interface constituting the concave inthe form of the inverted regular hexagonal cone.

As illustrated in FIGS. 14(a) and (b), in the M-cross section, an angleof the inclined interface in the first layer with respect to the mainsurface of the base substrate was about 47° or less. Further, asillustrated in FIG. 14(a), there were many inclined interfaces havingthe angle of about 47°.

On the other hand, the a-cross section (cross section in the directionalong the <1-100> axis) illustrated in FIGS. 15(a) and (b) was cut alongthe ridge line constituting the concave in the form of the invertedregular hexagonal cone.

As illustrated in FIGS. 15(a) and (b), in the a-cross section, most ofthe angles of the ridge lines constituting the concaves in the form ofthe inverted regular hexagonal cones with respect to the main surface ofthe base substrate, were about 43°. When the angle of the ridgeline isgeometrically calculated under the condition that the angle of theinclined interface constituting the inverted regular hexagonal cone is47°, the angle of the ridgeline is 43°. Therefore, it was confirmed thatthe angle of the inclined interface was about 47° in many concaves, evenbased on the angle of the ridgeline obtained from FIGS. 15(a) and 15(b).

Here, the angle of {11-2m} with respect to the {0001} plane of GaN is asfollows.

{11-21} plane: 72.9°{11-22} plane: 58.4°{11-23} plane: 47.3°{11-24} plane: 39.1°

As described above, it was confirmed that the inclined interfacegenerated on the surface of the first layer grown under the condition ofexperiment 2, was {11-2 m} plane satisfying m≥3. It was also confirmedthat most of the inclined interfaces were {11-23} planes.

According to experiment 2, since the first growth condition was adjustedso as to satisfy the formula (1) using the above-described basesubstrate in the same manner as in experiment 1, the {11-2m} planesatisfying m≥3 could be generated as the inclined interface. Thereby, itwas confirmed that the average distance between the closest tops couldbe more than 100 μm in the first layer.

(3) Experiment 3 (3-1) Preparation of a Nitride Semiconductor Substrate

In order to compare the in-plane distribution of the dislocation-freeregion and the in-plane distribution in the X-ray locking curvemeasurement, the following samples 1 to 3 were prepared. The substrateof sample 1 is a substrate corresponding to the substrate of the exampleof experiment 1. The substrate of sample 2 is a substrate obtained froma crystal layer in which a thick film is grown with the c-plane as agrowth surface. Further, the substrate of sample 3 is a substrateprepared by the conventional VAS method, and corresponds to the basesubstrate.

[Method for Preparing a Nitride Semiconductor Substrate of Sample 1]

The substrate of sample 1 was prepared by the same method as in theexample of experiment 1. For sample 1, the radius of curvature of thec-plane and the dislocation density were the same as those of thesubstrate of the example of experiment 1, except that an absolute valueof the off-angle and an off-direction were different from the substrateof the example of experiment 1.

[Conditions for Preparing a Nitride Semiconductor Substrate of Sample 2](Base Substrate) Material: GaN

Preparation method: VAS method

Diameter: 62 mm Thickness: 400 μm

Crystal plane with a lowest index closest to the main surface: c planeOff-angle: 0.5° in m-axis directionNo pattern processing such as mask layer on the main surface.(Crystal layer)

Material: GaN

Growth method: HVPE methodGrowth temperature: 1050° C.V/III ratio: 2.8Growth time: 15 hours(processing)Grinding: A cylindrical region was removed to obtain a cylindricalregion with a diameter of 56 mm.Slice: 630 μm thick, 5 sheetsBeveling processing: The diameter was 50.8 mm.Polishing: 400-450 μm thick.

[Conditions for Preparing a Nitride Semiconductor Substrate of Sample 3]

The substrate of sample 3 was prepared by the conventional VAS methodsimilar to that of the base substrate. For sample 3, the radius ofcurvature of the c-plane and the dislocation density were the same asthose of the base substrate, except that an absolute value of anoff-angle and an off-direction were different from those of the basesubstrate.

(3-2) Evaluation (Observation Using a Multiphoton Excitation Microscope)

Under the same conditions as in experiment 1, the main surfaces of thesubstrates of samples 1 to 3 were observed using a multiphotonexcitation microscope.

(X-Ray Locking Curve Measurement)

For each of the substrates of samples 1 to 3, two types of X-ray lockingcurve measurements similar to those in experiment 1 were performed. Atthis time, the measurement was performed at a plurality of measurementpoints set at 5 mm intervals in each of the m-axis direction and thea-axis direction in the main surface. Thereby, the ratio of FWHMa−FWHMbto FWHMa was obtained at a plurality of measurement points of eachsample.

(3-3) Result (3-3-1) In-Plane Distribution of Dislocation-Free Regions

A distribution of a dislocation-free region in the substrates of samples1 and 2 will be described with reference to FIGS. 16 to 33. FIGS. 16 to32 are views of the main surface of the substrate of sample 1 observedusing a multiphoton excitation microscope. In FIGS. 16 to 32, (x, y)indicates the coordinates in the m-axis direction and the coordinates inthe a-axis direction. FIG. 33 is a view of observing the main surface ofthe substrate of sample 2 using a multiphoton excitation microscope. InFIGS. 16 to 33, a thick line frame indicates a dislocation-free regionof 50 μm square.

[Sample 2]

In the substrate of sample 2 obtained from the crystal layer grown thickwith the c-plane as a growth surface, the dislocation density is loweredin inverse proportion to the thickness of the crystal layer, andtherefore an average dislocation density was 6.3×10⁵ cm⁻².

However, as illustrated in FIG. 33, in the substrate of sample 2, thedislocations were uniformly dispersed in the surface. The distributionof the dislocations was similar to that in FIG. 33 even in a region notshown. Therefore, the size of the dislocation-free region was smallerthan 50 μm square, and the dislocation-free region of 50 μm square wasnot formed, over an entire body of the substrate of sample 2.

Thus, even when the method of sample 2 for obtaining a high-qualitysubstrate is used as the conventional method, no dislocation-free regionof 50 μm square was formed on the obtained substrate. Therefore, it isconsidered that the dislocation-free region of 50 μm square is notformed even in a substrate prepared by another conventionalmanufacturing method in which a special step of collecting dislocationsis not performed.

[Sample 1]

In contrast, as illustrated in FIGS. 16 to 33, the main surface of thesubstrate of sample 1 includes a dislocation-free region of at least 50μm square. Further, in the substrate of sample 1, the dislocation-freeregions of 50 μm square were scattered over the entire main surface.

Further, at least one 50 μm square dislocation-free region was presentin all the 250 μm square visual fields illustrated in FIGS. 16 to 33.The main surface of the substrate of sample 1 includes non-overlapping50 μm square dislocation-free regions at a density of 1600/cm² or more.Specifically, the density of the non-overlapping 50 μm squaredislocation-free regions in the main surface of the substrate of sample1 was approximately 5200/cm².

As described above, according to sample 1, since the first growthcondition is adjusted so as to satisfy the formula (1) using theabove-described base substrate, the average distance between the closesttops could be more than 100 μm. Thereby, it was confirmed that thedislocation density in the main surface of the substrate could besufficiently lowered. It was also confirmed that since the averagedistance between the closest tops was more than 100 μm, thedislocation-free region of at least 50 μm square could be formed, andthe dislocation-free region could be scattered over the entire mainsurface. Further, it was also confirmed that the density of thenon-overlapping dislocation-free regions of 50 μm square in the mainsurface could be 1600/cm² or more.

(3-3-2) In-Plane Distribution in the X-Ray Locking Curve Measurement

The results of samples 1 to 3 are shown in tables 2, 3 and 4,respectively. In the table below, “difference” means (FWHMa−FWHMb)/FWHMa(%).

TABLE 2 <Sample 1> slit = 0.1 slit = open (mm) ω (deg.) FWHMb(sec.) (mm)ω (deg.) FWHMa(sec.) difference //M −20 16.97834 26.9 −20 16.97833 31.113.5% −15 16.98569 27.3 −15 16.98558 32.1 15.0% −10 16.99292 28 −1016.9926 31.7 11.7% −5 16.99957 28 −5 16.99923 32.9 14.9% 0 17.00571 27 017.00539 28.8 6.3% 5 17.00978 26.7 5 17.00962 29 7.9% 10 17.01599 26.910 17.01568 29.6 9.1% 15 17.02317 27.7 15 17.02288 31.1 10.9% 2017.03257 28.7 20 17.03206 39.9 28.1% //a −20 17.23878 28.5 −20 17.2385935.6 19.9% −15 17.2487 27.9 −15 17.24847 33.7 17.2% −10 17.25665 27.4−10 17.25634 32 14.4% −5 17.26536 27.9 −5 17.26474 33.3 16.2% 0 17.2706326.4 0 17.27034 29.1 9.3% 5 17.27604 26.3 5 17.27594 29.6 11.1% 1017.28341 26.2 10 17.28324 30.8 14.9% 15 17.29102 27.6 15 17.29078 31.813.2% 20 17.29929 27.5 20 17.29909 31.7 13.2%

TABLE 3 <Sample 2> slit = 0.1 slit = open (mm) ω (deg.) FWHMb(sec.) (mm)ω (deg.) FWHMa(sec.) difference //M −20 17.58718 30.6 −20 17.58637 6149.8% −15 17.60427 29.9 −15 17.60367 49 39.0% −10 17.62154 29.4 −1017.62094 53.2 44.7% −5 17.63899 29.2 −5 17.63827 51.6 43.4% 0 17.6562628.1 0 17.65561 52.3 46.3% 5 17.67447 29.4 5 17.5738 55.2 45.7% 1017.6939 29.6 10 17.6931 56.5 47.6% 15 17.71332 28.2 15 17.71235 59.252.4% 20 17.73509 28.9 20 17.73384 68.2 57.6% //a −20 17.19294 36.3 −2017.19209 75.8 52.1% −15 17.21843 33.1 −15 17.21749 75.9 56.4% −1017.24258 31.4 −10 17.24179 68.4 54.1% −5 17.26492 29.8 −5 17.26405 67.155.6% 0 17.28636 31.4 0 17.28551 67.3 53.3% 5 17.30887 31.4 5 17.3079469.2 54.6% 10 17.3332 33.6 10 17.33179 78.2 57.0% 15 17.35942 32.9 1517.35821 79.7 58.7% 20 17.385 30.5 20 17.38469 75.4 59.5%

TABLE 4 <Sample 3>: corresponding to base substrate slit = 0.1 slit =open (mm) ω (deg.) FWHMb(sec.) (mm) ω (deg.) FWHMa(sec.) difference //M−20 17.07689 42.9 −20 17.07888 187.2 77.1% −15 17.13755 45.6 −1517.14011 184 75.2% −10 17.19635 46.6 −10 17.19869 171.7 72.9% −517.25394 46.3 −5 17.25502 170.3 72.8% 0 17.31046 46.1 0 17.31278 170.372.9% 5 17.36629 44.2 5 17.36438 169.1 73.9% 10 17.42552 44.1 1017.42475 175.2 74.8% 15 17.48624 45.6 15 17.48797 179.6 74.6% 20 17.553443.9 20 17.5618 208.5 78.9% //a −20 17.14414 47.3 −20 17.14824 211.277.6% −15 17.20841 46.2 −15 17.20506 183.2 74.8% −10 17.27142 47.2 −1017.27079 180.1 73.8% −5 17.33018 50.5 −5 17.3292 176.7 71.4% 0 17.3881747.4 0 17.38517 169 72.0% 5 17.44551 50.2 5 17.44218 165.6 69.9% 1017.50449 46.5 10 17.50376 172.9 73.1% 15 17.56355 43.9 15 17.5611 17374.6% 20 17.62587 46.3 20 17.62643 178.5 74.1%

[Sample 3]

As shown in table 4, in the substrate of sample 3 prepared by theconventional VAS method, the variation in the off-angle of the c-axiswithin the diameter of 40 mm was about ±0.24°. Further, in the substrateof sample 3, FWHMb was more than 32 arcsec at all measurement points.Further, in the substrate of sample 3, (FWHMa−FWHMb)/FWHMa was more than30% at all measurement points.

[Sample 2]

As shown in table 3, in the substrate of sample 2 obtained from thecrystal layer in which a thick film was grown with the c-plane as agrowth surface, the variation in the off-angle of the c-axis within thediameter of 40 mm was improved as compared with the substrate of sample3, and was about ±0.074°. Further, FWHMb of the substrate of sample 2was improved as compared with FWHMb of the substrate of sample 3.

However, in the substrate of sample 2, a plurality of portions havingFWHMb exceeding 32 arcsec were found. Further, in the substrate ofsample 2, (FWHMa−FWHMb)/FWHMa greatly exceeded 30% at all measurementpoints.

As described above, the high-quality substrate of sample 2 as aconventional substrate has improved dislocation density and off-anglevariation as compared with the base substrate, but the substrate ofsample 2 did not have any point satisfying the full width at halfmaximum condition of FWHMb<32 arcsec and (FWHMa−FWHMb)/FWHMa≤30%. Thereason is considered as follows: in the substrate of sample 2, at leastone of the above-described crystal quality factors was not as good asthat of the substrate of sample 1.

Therefore, even the substrate of sample 2, which has a relatively highquality as a conventional substrate, does not satisfy theabove-described full width at half maximum condition, and therefore itis considered that the substrate prepared by other conventionalmanufacturing methods does not satisfy the above-described full width athalf maximum condition.

[Sample 1]

In contrast, as shown in table 2, in the substrate of sample 1, thevariation in the off-angle of the c-axis within the diameter of 40 mmwas smaller than that of the substrates of samples 2 and 3, and wasabout ±0.03°.

Further, in the substrate of sample 1, FWHMb was 32 arcsec or less atall measurement points. Further, in the substrate of sample 1,(FWHMa-FWHMb)/FWHMa was 30% or less at all measurement points.

As described above, in the substrate of sample 1 obtained by theabove-described manufacturing method, the dislocation density could belowered, and not only was it possible to reduce the off-angle variation,but it was also possible to improve all of the above-described crystalquality factors that determine the full width at half maximum in awell-balanced manner. Thereby, in the substrate of sample 1, it wasconfirmed that FWHMb could be 32 arcsec or less. Further, in sample 1,even when the slit width is 1 mm, the radius of curvature of the c-planeis large, and the above-described crystal quality factors arewell-balanced and good, over the entire region irradiated with X-rays,and therefore it was confirmed that (FWHMa−FWHMb)/FWHMa could be 30% orless.

<Preferable Aspects of the Present Disclosure>

Hereinafter, preferable aspects of the present disclosure will besupplementarily described.

(Supplementary Description 1)

There is provided a method for manufacturing a nitride semiconductorsubstrate using a vapor deposition method, including:

a step of preparing a base substrate comprising a single crystal of agroup III nitride semiconductor, and having a mirror main surface whoseclosest low index plane is a (0001) plane;

a first step of epitaxially growing a single crystal of a group IIInitride semiconductor having a top surface with (0001) plane exposed,directly on the main surface of the base substrate, forming a pluralityof concaves composed of inclined interfaces other than the (0001) planeon the top surface, gradually expanding the inclined interfaces towardan upper side of the main surface of the base substrate, making the(0001) plane disappear from the top surface, and growing a first layerwhose surface is composed only of the inclined interfaces; and

a second step of epitaxially growing a single crystal of a group IIInitride semiconductor on the first layer, making the inclined interfacesdisappear, and growing a second layer having a mirror surface,

wherein in the first step, a plurality of valleys and a plurality oftops are formed on a surface of the first layer by forming the pluralityof concaves on the top surface comprising a single crystal and makingthe (0001) plane disappear, and

when observing an arbitrary cross section perpendicular to the mainsurface,

an average distance between a pair of tops separated in a directionalong the main surface is more than 100 μm, the pair of tops beingclosest to each other among the plurality of tops, with one of theplurality of valleys sandwiched between them.

(Supplementary Description 2)

There is provided the method for manufacturing a nitride semiconductorsubstrate according to supplementary description 1,

wherein in the step of preparing the base substrate, root mean squareroughness of the main surface of the base substrate is 1 nm or more.

(Supplementary Description 3)

There is provided the method for manufacturing a nitride semiconductorsubstrate according to supplementary description 1 or 2,

wherein in the step of preparing the base substrate,

a crystal strain introduced by processing of the base substrate is lefton the main surface side of the base substrate,

full width at half maximum of (10-10) plane diffraction when X-raylocking curve measurement is performed with an incident angle of thebase substrate after processing set as 2° with respect to the mainsurface, is made larger than full width at half maximum of the basesubstrate before processing, and is set as 60 arcsec or more and 200arcsec or less.

(Supplementary Description 4)

There is provided the method for manufacturing a nitride semiconductorsubstrate according to any one of supplementary descriptions 1 to 3,wherein in the first step, an average distance between the pair of topsclosest to each other is less than 800 μm.

(Supplementary Description 5)

There is provided the method for manufacturing a nitride semiconductorsubstrate according to any one of supplementary descriptions 1 to 4,wherein in the first step, after making the (0001) plane disappear fromthe surface, growth of the first layer is continued over a predeterminedthickness while maintaining a state in which the surface is composedonly of the inclined interface.

(Supplementary Description 6)

There is provided the method for manufacturing a nitride semiconductorsubstrate according to any one of supplementary descriptions 1 to 5,including a step of slicing at least one nitride semiconductor substratefrom the second layer, after the second step.

(Supplementary Description 7)

There is provided the method for manufacturing a nitride semiconductorsubstrate according to any one of supplementary descriptions 1 to 6,

wherein in the step of preparing the base substrate,

the base substrate is prepared, whose (0001) plane is curved in aconcave spherical shape with respect to the main surface, and

in the step of slicing the nitride semiconductor substrate, a variationin an off-angle, which is an angle formed by <0001> axis with respect toa normal of the main surface of the nitride semiconductor substrate, issmaller than a variation in an off-angle, which is an angle formed by<0001> axis with respect to a normal of the main surface of the basesubstrate.

(Supplementary Description 8)

There is provided the method for manufacturing a nitride semiconductorsubstrate according to any one of supplementary descriptions 1 to 7,

wherein in the first step, the first layer is grown under a first growthcondition satisfying formula (1), and

in the second step, the second layer is grown under a second growthcondition satisfying formula (2),

G _(c1) >Gi/cos α  (1)

G _(c2) <Gi/cos α  (2)

(wherein, G_(c1) is a growth rate of the (0001) plane of the firstlayer, G_(c2) is a growth rate of the (0001) plane of the second layer,G_(i) is a growth rate of the inclined interface most inclined withrespect to the (0001) plane in each of the first layer and the secondlayer, and α is an angle formed by the (0001) plane and the inclinedinterface most inclined with respect to the (0001) plane in each of thefirst layer and the second layer.)

(Supplementary Description 9)

There is provided the method for manufacturing a nitride semiconductorsubstrate according to any one of supplementary descriptions 1 to 8,

wherein in the first step, a first c-plane growth region grown with the(0001) plane as a growth surface is formed in the first layer,

a convex portion is formed at a position where the (0001) planedisappears and terminates as an inflection point that is convex upwardin the first c-plane growth region,

a pair of inclined portions are formed on both sides of the firstc-plane growth region interposing the convex portion, as a locus of anintersection between the (0001) plane and the inclined interface, and

an angle formed by the pair of inclined portions is 70° or less.

(Supplementary Description 10)

There is provided the method for manufacturing a nitride semiconductorsubstrate according to any one of supplementary descriptions 1 to 9,

wherein the first step includes:

a step of gradually expanding the inclined interface toward an upperside of the base substrate to form an expanded inclined interface layer;and

a step of forming an inclined interface maintenance layer over apredetermined thickness on the expanded inclined interface layer inwhich the (0001) plane disappears from the surface, while maintaining astate where the surface is composed only of the inclined interface otherthan the (0001) plane.

(Supplementary Description 11)

There is provided the method for manufacturing a nitride semiconductorsubstrate according to any one of supplementary descriptions 1 to 10,

wherein the second step includes:

a step of contracting the inclined interface other than the (0001) planewhile expanding the (0001) plane toward an upper side of the firstlayer, to form a c-plane expanded layer; and

a step of forming a main growth layer over a predetermined thicknesswith the (0001) plane as a growth surface on the c-plane expanded layerwhose surface is mirror-finished.

(Supplementary Description 12)

There is provided the method for manufacturing a nitride semiconductorsubstrate according to any one of supplementary descriptions 1 to 11,

wherein in the first step,

{11-2m} plane satisfying m≥3 is generated as the inclined interface.

(Supplementary Description 13)

There is provided a nitride semiconductor substrate having a diameter of2 inches or more and having a main surface whose closest low indexcrystal plane is a (0001) plane,

wherein X-ray locking curve measurement for (0002) plane diffraction,which is performed to the main surface by irradiating with (Cu) Kα1X-rays through a two-crystal monochromator of Ge (220) plane and a slit,reveals that:

FWHMb is 32 arcsec or less,

difference FWHMa−FWHMb obtained by subtracting FWHMb from FWHMa is 30%or less of FWHMa, and

wherein FWHMa is full width at half maximum of the (0002) planediffraction when a slit width in ω direction is 1 mm, and

FWHMb is full width at half maximum of the (0002) plane diffraction whena slit width in ω direction is 0.1 mm, and

a diffraction pattern when the slit width in the ω direction is 1 mm hasa single peak.

(Supplementary Description 14)

There is provided the nitride semiconductor substrate according tosupplementary description 13, wherein X-ray locking curve measurement ofthe (0002) plane diffraction with the slit width in ω direction set as0.1 mm, which is performed at a plurality of measurement points set atintervals of 5 mm in the main surface, reveals that full width at halfmaximum FWHMb of the (0002) plane diffraction is 32 arcsec or less at90% or more of all measurement points.

(Supplementary Description 15)

There is provided the nitride semiconductor substrate according tosupplementary description 13 or 14, wherein observation of the mainsurface in a field of view of 250 μm square using a multiphotonexcitation microscope to obtain a dislocation density from a dark spotdensity, reveals that a region having a dislocation density of more than3×10⁶ cm⁻² does not exist in the main surface, and a region having adislocation density of less than 1×10⁶ cm⁻² exists in 80% or more of themain surface.

(Supplementary Description 16)

There is provided the nitride semiconductor substrate according to anyone of supplementary descriptions 13 to 15, wherein the main surfaceincludes non-overlapping 50 μm square dislocation-free regions at adensity of 100/cm² or more.

(Supplementary Description 17)

There is provided a nitride semiconductor substrate having a diameter of2 inches or more and having a main surface whose closest low indexcrystal plane is a (0001) plane,

wherein observation of the main surface of the nitride semiconductorsubstrate using a multiphoton excitation microscope in a field of viewof 250 μm square to obtain a dislocation density from a dark spotdensity, reveals that a region having a dislocation density of more than3×10⁶ cm⁻² does not exist in the main surface, and a region having adislocation density of less than 1×10⁶ cm⁻² exists in 80% or more of themain surface, and a region having a dislocation density of less than1×10⁶ cm⁻² exists in 80% or more of the main surface, and

the main surface has non-overlapping 50 μm square dislocation-freeregions at a density of 100/cm² or more.

(Supplementary Description 18)

There is provided the nitride semiconductor substrate according to anyone of claims 13 to 17, wherein oxygen concentration is 5×10¹⁶ cm⁻³ orless.

(Supplementary Description 19)

There is provided the nitride semiconductor substrate according to anyone of claims 13 to 18, wherein hydrogen concentration is less than1×10¹⁷ cm⁻³.

(Supplementary Description 20)

There is provided the nitride semiconductor substrate according to anyone of claims 13 to 19, wherein random extraction of 100 dislocations inthe main surface revels that a percentage of dislocations whose Burgersvector is either <11-20>/3, <0001> or <11-23>/3, is 50% or more.

(Supplementary Description 21)

There is provided a laminated structure, including:

a base substrate comprising a single crystal of a group III nitridesemiconductor and having a mirror main surface whose closest low indexcrystal plane is a (0001) plane;

a first low oxygen concentration region provided directly on the mainsurface of the base substrate and comprising a single crystal of a groupIII nitride semiconductor;

a high oxygen concentration region provided on the first low oxygenconcentration region and comprising a single crystal of a group IIInitride semiconductor; and

a second low oxygen concentration region provided on the high oxygenconcentration region and comprising a single crystal of a group IIInitride semiconductor,

wherein an oxygen concentration in the high oxygen concentration regionis higher than an oxygen concentration in each of the first low oxygenconcentration region and the second low oxygen concentration region, and

when observing an arbitrary cross section perpendicular to the mainsurface,

a top surface of the first low oxygen concentration region has aplurality of valleys and a plurality of mountains, and

an average distance between a pair of mountains separated in a directionalong the main surface is more than 100 μm, the pair of mountains beingclosest to each other among the plurality of mountains, with one of theplurality of valleys sandwiched between them.

(Supplementary Description 22)

There is provided the laminated structure according to the supplementarydescription 21, wherein the high oxygen concentration region iscontinuously provided along the main surface of the base substrate.

(Supplementary Description 23)

There is provided the laminated structure according to the supplementarydescription 21 or 22, wherein the first low oxygen concentration regionhas a pair of inclined portions provided on both sides of the mountain,and an angle formed by the pair of inclined portions is 70° or less.

(Supplementary Description 24)

There is provided the laminated structure according to any one of thesupplementary descriptions 21 to 23,

wherein a reduction rate of a dislocation density obtained by N/N₀ issmaller than a reduction rate of a dislocation density obtained byN′/N₀,

wherein the dislocation density in the main surface of the basesubstrate is N₀ and the dislocation density in a boundary surface alongthe main surface at an upper end of the high oxygen concentration regionis N, and the dislocation density in the surface of a crystal layer isN′ when a crystal layer of a group III nitride semiconductor isepitaxially grown on the main surface of the base substrate to athickness equal to a thickness from the main surface to the boundarysurface of the base substrate, with only the (0001) plane as a growthsurface.

(Supplementary Description 25)

There is provided the laminated structure according to any one of thesupplementary descriptions 21 to 24,

wherein a thickness of a boundary surface along the main surface at anupper end of the high oxygen concentration region, from the main surfaceof the base substrate is 1.5 mm or less, and

a reduction rate of the dislocation density obtained by N/N₀ is 0.3 orless, wherein the dislocation density in the main surface of the basesubstrate is N₀, and the dislocation density in the boundary surface isN.

DESCRIPTION OF SIGNS AND NUMERALS

-   10 Base substrate-   30 First layer-   40 Second layer-   50 Nitride semiconductor substrate (substrate)

1. A method for manufacturing a nitride semiconductor substrate using avapor deposition method, including: a step of preparing a base substratecomprising a single crystal of a group III nitride semiconductor, andhaving a mirror main surface whose closest low index plane is a (0001)plane; a first step of epitaxially growing a single crystal of a groupIII nitride semiconductor having a top surface with (0001) planeexposed, directly on the main surface of the base substrate, forming aplurality of concaves composed of inclined interfaces other than the(0001) plane on the top surface, gradually expanding the inclinedinterfaces toward an upper side of the main surface of the basesubstrate, making the (0001) plane disappear from the top surface, andgrowing a first layer whose surface is composed only of the inclinedinterfaces; and a second step of epitaxially growing a single crystal ofa group III nitride semiconductor on the first layer, making theinclined interfaces disappear, and growing a second layer having amirror surface, wherein in the first step, a plurality of valleys and aplurality of tops are formed on a surface of the first layer by formingthe plurality of concaves on the top surface comprising a single crystaland making the (0001) plane disappear, and when observing an arbitrarycross section perpendicular to the main surface, an average distancebetween a pair of tops separated in a direction along the main surfaceis more than 100 μm, the pair of tops being closest to each other amongthe plurality of tops, with one of the plurality of valleys sandwichedbetween them.
 2. The method for manufacturing a nitride semiconductorsubstrate according to claim 1, wherein in the step of preparing thebase substrate, root mean square roughness of the main surface of thebase substrate is 1 nm or more.
 3. The method for manufacturing anitride semiconductor substrate according to claim 1, wherein in thestep of preparing the base substrate, crystal strain introduced byprocessing of the base substrate is left on the main surface side of thebase substrate, and full width at half maximum of (10-10) planediffraction when X-ray locking curve measurement is performed with anincident angle of the base substrate after processing set as 2° withrespect to the main surface, is made larger than full width at halfmaximum of the base substrate before processing, and is set as 60 arcsecor more and 200 arcsec or less.
 4. The method for manufacturing anitride semiconductor substrate according to claim 1, wherein theaverage distance between the pair of tops that are closest to each otheris less than 800 μm.
 5. The method for manufacturing a nitridesemiconductor substrate according to claim 1, wherein in the first step,after the (0001) plane disappears from the surface, growth of the firstlayer is continued over a predetermined thickness while maintaining astate where the surface is composed only of the inclined interfaces. 6.The method for manufacturing a nitride semiconductor substrate accordingto claim 1, wherein after the second step, there is a step of slicing atleast one nitride semiconductor substrate from the second layer.
 7. Themethod for manufacturing a nitride semiconductor substrate according toclaim 6, wherein in the step of preparing the base substrate, the basesubstrate is prepared, whose (0001) plane is curved in a concavespherical shape with respect to the main surface, and in the step ofslicing the nitride semiconductor substrate, a variation in anoff-angle, which is an angle formed by <0001> axis with respect to anormal of the main surface of the nitride semiconductor substrate, issmaller than a variation in an off-angle, which is an angle formed by<0001> axis with respect to a normal of the main surface of the basesubstrate.
 8. The method for manufacturing a nitride semiconductorsubstrate according to claim 1, wherein in the first step, a firstc-plane growth region grown with the (0001) plane as a growth surface isformed in the first layer, a convex portion is formed at a positionwhere the (0001) plane disappears and terminates as an inflection pointthat is convex upward in the first c-plane growth region, a pair ofinclined portions are formed on both sides of the first c-plane growthregion interposing the convex portion, as a locus of an intersectionbetween the (0001) plane and the inclined interface, and an angle formedby the pair of inclined portions is 70° or less.
 9. The method formanufacturing a nitride semiconductor substrate according to claim 1,wherein in the first step, {11-2m} plane satisfying m≥3 is generated asthe inclined interface.
 10. A nitride semiconductor substrate, having adiameter of 2 inches or more and having a main surface whose closest lowindex crystal plane is a (0001) plane, wherein X-ray locking curvemeasurement for (0002) plane diffraction, which is performed to the mainsurface by irradiating with (Cu) Kα1 X-rays through a two-crystalmonochromator of Ge (220) plane and a slit, reveals that: FWHMb is 32arcsec or less, and difference FWHMa−FWHMb obtained by subtracting FWHMbfrom FWHMa is 30% or less of FWHMa, wherein FWHMa is full width at halfmaximum of the (0002) plane diffraction when a slit width in ω directionis 1 mm, FWHMb is full width at half maximum of the (0002) planediffraction when a slit width in ω direction is 0.1 mm, and adiffraction pattern when the slit width in ω direction is 1 mm has asingle peak.
 11. The nitride semiconductor substrate according to claim10, wherein X-ray locking curve measurement of the (0002) planediffraction with the slit width in ω direction set as 0.1 mm, which isperformed at a plurality of measurement points set at intervals of 5 mmin the main surface, reveals that full width at half maximum FWHMb ofthe (0002) plane diffraction is 32 arcsec or less at 90% or more of allmeasurement points.
 12. The nitride semiconductor substrate according toclaim 10, wherein observation of the main surface of the nitridesemiconductor substrate in a field of view of 250 μm square using amultiphoton excitation microscope to obtain a dislocation density from adark spot density, reveals that: there is no region in the main surfacewhere the dislocation density exceeds 3×10⁶ cm⁻², and a region having adislocation density of less than 1×10⁶ cm⁻² exists in an area of 80% ormore of the main surface.
 13. The nitride semiconductor substrateaccording to claim 10, wherein the main surface includes non-overlapping50 μm square dislocation-free regions at a density of 100/cm² or more.14. A nitride semiconductor substrate having a diameter of 2 inches ormore and having a main surface whose closest low index crystal plane isa (0001) plane, wherein observation of a main surface of the nitridesemiconductor substrate in a field of view of 250 μm square using amultiphoton excitation microscope to obtain a dislocation density from adark spot density, reveals that: there is no region in the main surfacewhere the dislocation density exceeds 3×10⁶ cm⁻², and a region having adislocation density of less than 1×10⁶ cm⁻² exists in an area of 80% ormore of the main surface, and the main surface includes non-overlapping50 μm square dislocation-free regions at a density of 100/cm² or more.15. The nitride semiconductor substrate according to claim 10, whereinoxygen concentration is 5×10¹⁶ cm⁻³ or less.
 16. A laminated structure,comprising: a base substrate comprising a single crystal of a group IIInitride semiconductor and having a mirror main surface whose closest lowindex crystal plane is a (0001) plane; a first low oxygen concentrationregion provided directly on the main surface of the base substrate andcomprising a single crystal of a group III nitride semiconductor; a highoxygen concentration region provided on the first low oxygenconcentration region and comprising a single crystal of a group IIInitride semiconductor; and a second low oxygen concentration regionprovided on the high oxygen concentration region and comprising a singlecrystal of a group III nitride semiconductor, wherein an oxygenconcentration in the high oxygen concentration region is higher than anoxygen concentration in each of the first low oxygen concentrationregion and the second low oxygen concentration region, and whenobserving an arbitrary cross section perpendicular to the main surface,a top surface of the first low oxygen concentration region has aplurality of valleys and a plurality of mountains, and an averagedistance between a pair of mountains separated in a direction along themain surface is more than 100 μm, the pair of mountains being closest toeach other among the plurality of mountains, with one of the pluralityof valleys sandwiched between them.
 17. The laminated structureaccording to claim 16, wherein the high oxygen concentration region iscontinuously provided along the main surface of the base substrate. 18.The laminated structure according to claim 16, wherein the first lowoxygen concentration region further has a pair of inclined portionsprovided on both sides of the mountain, and an angle formed by the pairof inclined portions is 70° or less.
 19. The laminated structureaccording to claim 16, wherein a reduction rate of a dislocation densityobtained by N/N₀ is smaller than a reduction rate of a dislocationdensity obtained by N′/N₀, wherein the dislocation density in the mainsurface of the base substrate is N₀ and the dislocation density in aboundary surface along the main surface at an upper end of the highoxygen concentration region is N, and the dislocation density in asurface of a crystal layer is N′ when the crystal layer of a group IIInitride semiconductor is epitaxially grown on the main surface of thebase substrate to a thickness equal to a thickness from the main surfaceto the boundary surface of the base substrate, with only a (0001) planeas a growth surface.
 20. The laminated structure according to claim 16,wherein a thickness of a boundary surface along the main surface at anupper end of the high oxygen concentration region, from the main surfaceof the base substrate is 1.5 mm or less, and a reduction rate of adislocation density obtained by N/N₀ is 0.3 or less, wherein thedislocation density in the main surface of the base substrate is N₀, andthe dislocation density in the boundary surface is N.